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Queries and Joins

This section describes how to query YugabyteDB using the YSQL SELECT statement and its clauses.

Querying Data

The main purpose of SELECT statements is to retrieve data from specified tables. Typically, the first part of every SELECT statement defines columns that contain the required data, the second part points to the tables hosting these columns, and the third, optional part, lists restrictions.

SELECT has the following syntax:

SELECT list FROM table_name;

list represents a column or a list of columns in a table_name table that is the subject of data retrieval. If you provide a list of columns, you need to use a comma separator between two columns. To select data from all the columns in the table, you can use an asterisk. list can also contain expressions or literal values.

Note that the FROM clause is evaluated before SELECT.

The following SELECT statement clauses provide flexiblity and allow you to fine-tune queries:

  • The DISTINCT operator allows you to select distinct rows.
  • The ORDER BY clause lets you sort rows.
  • The WHERE clause allows you to apply filters to rows.
  • The LIMIT clause allows you to select a subset of rows from a table.
  • The GROUP BY clause allows you to divide rows into groups.
  • The HAVING clause lets you filter groups.
  • The INNER JOIN, LEFT JOIN, FULL OUTER JOIN, and CROSS JOIN clauses let you create joins with other tables.

SELECT Examples

Suppose you work with a database that includes the following table populated with data:

CREATE TABLE employees (
  employee_no integer PRIMARY KEY,
  name text,
  department text
);

INSERT INTO employees VALUES
  (1221, 'John Smith', 'Marketing'),
  (1222, 'Bette Davis', 'Sales'),
  (1223, 'Lucille Ball', 'Operations'),
  (1224, 'John Zimmerman', 'Sales');

You can use the SELECT statement to find names of all employees in the employees table. In this case, you apply SELECT to one column only, as follows:

SELECT name FROM employees;

To retrieve data that includes the employee name and department, you need to query multiple columns, as shown in the following example:

SELECT name, department FROM employees;

To obtain data from all the columns in the table, you can use an asterisk, as shown in the following example:

SELECT * FROM employees;

You can also use expressions for data retrieval via SELECT. The following example shows how to use the concatenation operator to get employee numbers and names combined with departments for all employees:

SELECT employee_no, name || ' ' || department FROM employees;

The following is the output produced by the preceding example:

employee_no | ?column
------------+---------------------------
1221        | John Smith Marketing
1222        | Bette Davis Sales
1223        | Lucille Ball Operations
1224        | John Zimmerman Sales

In some cases, you may omit the FROM clause in the SELECT statement. The following example shows the SELECT statement that uses and expression to perform multiplication and outputs the result in a column:

SELECT 2 * 5;

You can always view your table definitions by executing the following command:

yugabyte=# \d employees

Column Aliases

You can use YSQL column aliases to provide meaningful column headers to a query output by assigning a temporary name to a column or an expression in the select list of your SELECT statement. An alias lifecycle ends as soon as the query finished executing.

A column alias has the following syntax:

SELECT column_name AS alias_name FROM table_name;

The AS keyword is optional; if omitted, the following syntax applies:

SELECT column_name alias_name FROM table_name;

The following syntax is used for setting an alias for an expression:

SELECT expression AS alias_name FROM table_name;

Using the table from SELECT Examples, the following example demonstrates how to retrieve data that includes the employee name and department:

SELECT name, department FROM employees;

The following example renames the department column to section using an alias:

SELECT name, department AS section FROM employees;

The following is the output produced by the preceding example:

name                | section
--------------------+---------------------------
John Smith          | Marketing
Bette Davis         | Sales
Lucille Ball        | Operations
John Zimmerman      | Sales

Column aliases may contain spaces. In this case, you enclose them in double quotes to produce multi-word headers, as shown in the following example:

SELECT name, department AS "section of the company" FROM employees;

Sorting and Ordering Data

The SELECT statement returns data in an unspecified order. You can use the SELECT statement's ORDER BY clause to sort the rows of the query result set in ascending or descending order based on a sort expression.

ORDER BY has the following syntax:

SELECT list
  FROM table_name
  ORDER BY sort_expression1 [ASC | DESC] [NULLS FIRST | NULLS LAST],
  ...,
  sort_expressionN [ASC | DESC];

sort_expression can be a column name or an expression that you intend to sort. To sort the query result set based on multiple columns or expressions, include a comma separator between two columns or expressions. Use the ASC option to sort rows in ascending order and DESC to sort rows in descending order; if you do not specify these options, ASC is used by default.

The ORDER BY clause is evaluated after FROM and SELECT. This gives you an opportunity to specify a column alias in the SELECT statement and use this alias in the ORDER BY clause.

Using the table from SELECT Examples, the following example demonstrates how sort employees based on their name in ascending order:

SELECT name, department FROM employees ORDER BY name DESC;

The following is the output produced by the preceding example:

name                | department
--------------------+---------------------------
Lucille Ball        | Operations
John Smith          | Marketing
John Zimmerman      | Sales
Bette Davis         | Sales

Omitting the DESC option would result in the employees sorted in ascending order by their name.

The following example selects the name and department from the employees table, then sorts the rows by the name in ascending order, and then sorts the already sorted rows by department in descending order:

SELECT name, department FROM employees
  ORDER BY name ASC, department DESC;

The following is the output produced by the preceding example:

name                | department
--------------------+---------------------------
Bette Davis         | Sales
John Zimmerman      | Sales
John Smith          | Marketing
Lucille Ball        | Operations

Sorting rows that contain NULL is typically done by using the ORDER BY clause's NULLS FIRST and NULLS LAST options. This allows you to specify the order of NULL with other non-null values: NULLS FIRST places NULL before other non-null values, whereas NULL LAST places NULL after other non-null values.

Which NULL option of the ORDER BY clause is used by default depends on whether a DESC or ASC option is specified: the ORDER BY clause with the DESC option uses the NULLS FIRST by default, and the ORDER BY clause with the ASC option uses the NULLS LAST by default.

The following example demonstrates how to sort the employees table by department in ascending order displaying rows with missing departments first:

SELECT department FROM employees
  ORDER BY department ASC NULLS FIRST;

Duplicate Rows

You can use the DISTINCT clause in the SELECT statement to remove duplicate rows from a query result. The DISTINCT clause keeps one row for each set of duplicates. You can apply this clause to columns included in the SELECT statement's select list.

The DISTINCT clause has the following syntax, with values in column_name evaluating the duplicate:

SELECT DISTINCT column_name FROM table_name;

In cases when multiple columns are used, the DISTINCT clause combines values of these columns to evaluate the duplicate.

Since the order of rows returned by the SELECT statement is unspecified, the first row of each set of duplicates is unknown. DISTINCT ON (expression) allows you to keep the first row of each set of duplicates. using the following syntax:

The DISTINCT ON (expression) clause has the following syntax:

SELECT DISTINCT ON (column_name_1) column_alias, column_name_2
  FROM table_name
  ORDER BY column_name_1, column_name_2;

The following series of examples inserts new rows into the table from SELECT Examples, then queries the employees table using SELECT with its DISTINCT option enabled, thus removing duplicate values, and then sorts the result set in descending order based on the employee name:

INSERT INTO employees (employee_no, name, department)
VALUES
(9, 'Jean Harlow', 'Sales'),
(8, 'Jean Harlow', 'Sales');

SELECT DISTINCT name FROM employees ORDER BY name DESC;

The following is the output produced by the preceding examples:

name
--------------------
Lucille Ball
John Smith
John Zimmerman
Jean Harlow
Bette Davis

Case Sensitivity

YSQL converts identifiers to lowercase unless they are enclosed in quotation marks. That is, YSQL is case-insensitive for all practical purposes by default. For example, a table called Employees would be recognized as the employees table and the following query would be executed on the employees table without any problems:

SELECT name FROM Employees;

The following example shows how to run a query specifically on a table called Employees:

SELECT name FROM "Employees";

Filtering Data

The WHERE clause allows you to filter data returned by the SELECT statement. Only the rows that satisfy a specified condition are included in the result set.

The WHERE clause has the following syntax:

SELECT list FROM table_name
  WHERE condition
  ORDER BY expression;

condition is a boolean expression or a combination of boolean expressions created with AND and OR logical operators. condition evaluates to TRUE, FALSE, or unknown. The result set only returns rows that cause condition to evaluate to TRUE.

The following example uses the table from SELECT Examples to demonstrate how to use the AND operator to combine two Boolean expressions in order to find an employee number of a specific employee working for a specified department:

SELECT employee_no FROM employees
  WHERE name = 'John Smith' AND department = 'Marketing';

The following is the output produced by the preceding example:

employee_no
--------------------
1221

The following example shows how to use the OR operator to find employee IDs of specific employees:

SELECT employee_no FROM employees
  WHERE name = 'John Smith' OR name = 'Bette Davis';

The following is the output produced by the preceding example:

employee_no
--------------------
1221
1222

During the query execution, the WHERE clause is evaluated after the FROM clause but before the SELECT and ORDER BY clause.

You cannot use column aliases in the WHERE clause of SELECT.

You can define the condition in the WHERE clause by using any standard SQL comparison operators and almost all logical operators except ALL, ANY, and SOME.

To find a string that matches a specified pattern, you use the LIKE operator. The following example returns all customers whose first names start with the string Ann:

The following example shows how to use the LIKE operator to find a string that matches a specified pattern returning employees:

SELECT name, department FROM employees WHERE name LIKE 'John%';

The following is the output produced by the preceding example:

name            | department
----------------+--------------------
John Smith      | Sales
John Zimmerman  | Marketing

YSQL also allows you to use numeric expressions and dates in the WHERE clause, as shown in the following examples that use the table from SELECT Examples:

SELECT name FROM employees WHERE employee_no = 1000 + 222;

The following is the output produced by the preceding example:

name
--------------------
Bette Davis

If the employees table had an additional column for the current timestamp that records the time of changes to every row, then the following example could have demonstrated how to use date expressions in the WHERE clause:

SELECT name FROM employees
  WHERE CURRENT_TIMESTAMP = '2021-01-03 16:22:29.079+07:30';

If one of the employee records was last modified on specific date and time, then the following could be the output produced by the preceding example:

name
--------------------
John Smith

The LIMIT Clause

The LIMIT clause of the SELECT statement allows you to impose constrains on the number of rows that your query can return.

The LIMIT clause has the following syntax:

SELECT list FROM table_name
  ORDER BY expression
  LIMIT rows_number;

rows_number represents the number of rows included in the query result set. If this number is set to zero, the query does not return any rows. If rows_number is set to NULL, the query result is the same as if the SELECT statement did not contain the LIMIT clause.

To skip rows before returning the rows specified by rows_number, you can use the OFFSET clause immediately after the LIMIT clause, as per the following syntax:

SELECT list FROM table_name
  LIMIT rows_number OFFSET row_skip;

row_skip represents the number of rows that the query skips before returning rows_number rows. If row_skip is set to zero, the query result is the same as if the SELECT statement did not contain the OFFSET clause.

Since rows are often stored in tables in an unspecified order, it is recommended that you include the ORDER BY clause in SELECT statements that contain the LIMIT clause.

Using the table from SELECT Examples, the following example demonstrates how retrieve the first two employees sorted by their number:

SELECT employee_no, name FROM employees
  ORDER BY employee_no
  LIMIT 2;

The following is the output produced by the preceding example:

employee_no | name
------------+--------------------
1221        | John Smith
1222        | Bette Davis

The following example demonstrates how retrieve three employees starting from the second one ordered by their number:

SELECT name, department FROM employees
  ORDER BY name
  LIMIT 3
  OFFSET 2;

The following is the output produced by the preceding example:

name                | department
--------------------+----------------------
John Smith          | Marketing
John Zimmerman      | Sales
Lucille Ball        | Operations

The LIKE Operator

There are cases when you do not know the exact query parameter but have an idea of a partial parameter. Using the LIKE operator allows you to match this partial information with existing data based on a pattern recognition.

The following is the syntax of the LIKE operator:

value LIKE pattern

The expression evaluates to true if value matches pattern.

For example, if your goal is to find an employee and their department, yet you only know that the employee name starts with Luci, you can execute the following query on a table created in SELECT Examples:

SELECT name, department FROM employees WHERE name LIKE 'Luci%';

The following is the output produced by the preceding example:

name          | department
--------------+----------------
Lucille Ball  | Operations

The WHERE clause contains a special expression consisting of name, the LIKE operator, and a string that contains a percent sign. The string 'Luci%' represents a pattern.

Rows returned by the query are those whose values in the name column begin with Luci and might be followed by any other characters.

To construct a pattern, you combine literal values with wildcard characters such as percent sign or underscore and use the LIKE or NOT LIKE operator to search for matches. Percent sign enables you to find a match for any sequence of any number of characters, whereas underscore matches any single character.

If you do not provide a wildcard character in the pattern, the LIKE operator acts like the equal operator.

The following is the syntax of the NOT LIKE operator:

value NOT LIKE pattern

The NOT LIKE operator behaves as an opposite of the LIKE operator and returns true when value does not match the pattern.

Grouping Data

YSQL allows you to divide rows of the result set into groups using the GROUP BY clause of the SELECT statement. You can apply an aggregate function to each group to calculate the sum of items. You can also count items in a group using the COUNT() function.

The GROUP BY clause has the following syntax:

SELECT column_1, column_2, aggregate_function(column_3)
  FROM table_name
  GROUP BY column_1, column_2;

column_1 and column_2 in the SELECT part of the statement represent columns that are to be selected. column_3 represents a column to which an aggregate function is applied. In the second, GROUP BY part of the statement, column_1 and column_2 are the columns that you want to group.

The purpose of the statement clause is to divide the rows by the values of the columns specified in the GROUP BY clause and calculate a value for each group.

The GROUP BY clause is evaluated after the FROM and WHERE clauses but before the HAVING, SELECT, DISTINCT, ORDER BY, and LIMIT clauses.

Using the table from SELECT Examples, the following example demonstrates how retrieve data from a table and group the result by employee_no:

SELECT employee_no FROM employees GROUP BY employee_no;

If there were duplicate rows in the result set from the preceding example, these rows would have been removed because the GROUP BY clause functions like the DISTINCT clause in this case.

If the employees table had an amount column with the employee pay data, the following example would have demonstrated how to select the total amount that each employee was paid. The GROUP BY clause would have divided the rows in the employees table into groups by employee number. For each group, the total amounts would have been calculated using the SUM() function:

SELECT employee_no, SUM (amount) FROM employees GROUP BY employee_no;

The HAVING Clause

To define search condition for a group or an aggregate, you can use the HAVING clause. If you use this clause in combination with the GROUP BY clause, you can filter groups and aggregates based on the condition.

The HAVING clause has the following syntax:

SELECT column_1, aggregate_function(column_2)
  FROM table_name
  GROUP BY column_1
  HAVING condition;

The GROUP BY clause returns rows grouped by column_1. The HAVING clause specifies condition to filter the groups.

The HAVING clause is evaluated after the FROM, WHERE, GROUP BY, but before the SELECT, DISTINCT, ORDER BY, and LIMIT clauses. Since the HAVING clause is evaluated before the SELECT clause, you cannot use column aliases in the HAVING clause.

Using the table from SELECT Examples, the following example demonstrates how to select the department that has more than one employee:

SELECT department, COUNT (employee_no)
  FROM employees
  GROUP BY department
  HAVING COUNT (employee_no) > 1;

The following is the output produced by the preceding examples:

department  | count
------------+----------
Sales       | 2

Joining Columns

You can combine (join) columns from the same or different tables based on the values of the common columns between related tables. Typically, common columns contain primary keys in the first table and foreign key in the second table.

Cross join, inner join, right outer join, left outer join, and full outer join are all supported by YSQL.

Suppose you work with a database that includes two tables created and populated as follows:

CREATE TABLE fulltime_employees (
  ft_employee_no integer PRIMARY KEY,
  ft_name text,
  ft_department text
);

INSERT INTO fulltime_employees VALUES
(1221, 'John Smith', 'Marketing'),
(1222, 'Bette Davis', 'Sales'),
(1223, 'Lucille Ball', 'Operations'),
(1224, 'John Zimmerman', 'Sales');

CREATE TABLE permanent_employees (
  perm_employee_no integer  PRIMARY KEY,
  perm_name text,
  perm_department text
);

INSERT INTO permanent_employees VALUES
(1221, 'Lucille Ball', 'Operations'),
(1222, 'Cary Grant', 'Operations'),
(1223, 'John Smith', 'Marketing');

The following is the fulltime_employees table:

ft_employee_no  | ft_name           | ft_department
----------------+-------------------+-------------------
1221            | John Smith        | Marketing
1222            | Bette Davis       | Sales
1223            | Lucille Ball      | Operations
1224            | John Zimmerman    | Sales

The following is the permanent_employees table:

perm_employee_no  | perm_name           | perm_department
------------------+---------------------+----------------------
1221              | Lucille Ball        | Operations
1222              | Cary Grant          | Operations
1223              | John Smith          | Marketing

You can always view your table definitions by executing the following commands:

yugabyte=# \d fulltime_employees
yugabyte=# \d permanent_employees

Inner Join

The following example demonstrates how to join the fulltime_employees table with the permanent_employees table by matching the values in the ft_name and perm_name columns:

SELECT ft_employee_no, ft_name, perm_employee_no, perm_name
  FROM fulltime_employees
  INNER JOIN permanent_employees
  ON ft_name = perm_name;

The following is the output produced by the preceding examples:

ft_employee_no  | ft_name       | perm_employee_no | perm_name
----------------+---------------+------------------+----------------
1221            | John Smith    | 1223             | John Smith
1223            | Lucille Ball  | 1221             | Lucille Ball

Each row of the fulltime_employees table has been examined and the value in its ft_name column compared with the value in the perm_name column for each row in the permanent_employees table. In case of equal values, a new row was created and its columns populated by values from both tables, then this new row was added to the result set.

Left Outer Join

The following example demonstrates how to use the left join to join the fulltime_employees table (left table) with the permanent_employees table (right table):

SELECT ft_employee_no, ft_name, perm_employee_no, perm_name
  FROM fulltime_employees
  LEFT JOIN permanent_employees
  ON ft_name = perm_name;

The following is the output produced by the preceding examples:

ft_employee_no  | ft_name         | perm_employee_no  | perm_name
----------------+-----------------+-------------------+-----------------
1221            | John Smith      | 1223              | John Smith
1223            | Lucille Ball    | 1221              | Lucille Ball
1222            | Bette Davis     | [null]            | [null]
1224            | John Zimmerman  | [null]            | [null]

The statement execution starts by selecting data from the fulltime_employees table, values in its ft_name column are compared with the values in the perm_name column for each row in the permanent_employees table. In case of equal values, a new row is created and its columns populated by values from both tables, then this new row is added to the result set. When non-equal values are encountered, a new row is created containing columns from both tables, and then this new row is added to the result set. The columns of the right table permanent_employees are populated with null values.

The following example shows how to select the fulltime_employees table rows that do not have matching rown in the permanent_employees table:

SELECT ft_employee_no, ft_name, perm_employee_no, perm_name
  FROM fulltime_employees
  LEFT JOIN permanent_employees
  ON ft_name = perm_name
  WHERE perm_employee_no IS NULL;

The following is the output produced by the preceding examples:

ft_employee_no  | ft_name         | perm_employee_no  | perm_name
----------------+-----------------+-------------------+----------------
1222            | Bette Davis     | [null]            | [null]
1224            | John Zimmerman  | [null]            | [null]

Right Outer Join

Unlike the left join that starts data selection from the left table, the right join starts selecting data from the right table. It compares every value in the perm_name column of every row in the permanent_employees table (right table) with every value in the ft_name column of every row in the fulltime_employees table (left table). In case of equal values, a new row that contains columns from both tables is created. When non-equal values are encountered, an additional new row containing columns from both tables is created and columns of the left table fulltime_employees is populated with null values.

The following example demonstrates how to use the right join to join the fulltime_employees table with the permanent_employees table:

SELECT ft_employee_no, ft_name, perm_employee_no, perm_name
  FROM fulltime_employees
  RIGHT JOIN permanent_employees
  ON ft_name = perm_name;

The following is the output produced by the preceding examples:

ft_employee_no  | ft_name         | perm_employee_no  | perm_name
----------------+-----------------+-------------------+------------------
1223            | John Smith      | 1221              | John Smith
1221            | Lucille Ball    | 1223              | Lucille Ball
[null]          | [null]          | 1222              | Cary Grant

By adding a WHERE clause to the end of the SELECT statement, you can obtain rows from the right table that do not have matching rows in the left table.

Full Outer Join

The full outer join allows you to obtain a result set that contains all rows from left and right table, with the matching rows from both sides (if any). If no match exists, as in the following example, the left table's columns are populated with null values:

SELECT ft_employee_no, ft_name, perm_employee_no, perm_name
  FROM fulltime_employees
  FULL OUTER JOIN permanent_employees
  ON ft_name = perm_name;

The following is the output produced by the preceding examples:

ft_employee_no  | ft_name         | perm_employee_no  | perm_name
----------------+-----------------+-------------------+-----------------
1221            | John Smith      | 1223              | John Smith
1222            | Bette Davis     | 1221              | Lucille Ball
1223            | Lucille Ball    | [null]            | [null]
1224            | John Zimmerman  | [null]            | [null]
[null]          | [null]          | 1222              | Cary Grant

The following example shows how to use the full join with a WHERE clause to return rows in a table that do not have matching rows in another table:

SELECT ft_employee_no, ft_name, perm_employee_no, perm_name
  FROM fulltime_employees
  FULL JOIN permanent_employees
  ON ft_name = perm_name
  WHERE ft_employee_no IS NULL OR perm_employee_no IS NULL;

The following is the output produced by the preceding examples:

ft_employee_no  | ft_name         | perm_employee_no  | perm_name
----------------+-----------------+-------------------+----------------
1222            | Bette Davis     | [null]            | [null]
1224            | John Zimmerman  | [null]            | [null]
[null]          | [null]          | 1222              | Cary Grant

Cross Join

You can use a cross join to generate a Cartesian product of rows in at least two tables.

Unlike other join clauses, the CROSS JOIN clause does not have a join predicate.

The CROSS JOIN clause has the following syntax:

SELECT list FROM table_1_name
  CROSS JOIN table_2_name;

You may omit the CROSS JOIN clause and use the following syntax instead:

SELECT list FROM table_1_name, table_2_name;

As an alternative, you can use the following syntax to simulate the cross join by using an inner join with a condition which always evaluates to true:

SELECT * FROM table_1_name
  INNER JOIN table_2_name ON true;

The fulltime_employees table has 4 rows and the permanent_employees table has 3 rows. When these tables are cross-joined, the result set has 4 * 3 = 12 rows, as the following example demonstrates:

SELECT * FROM fulltime_employees
  CROSS JOIN permanent_employees;

Subqueries

Subqueries allow you to construct complex queries by executing SELECT, INSERT, DELETE or UPDATE statements from within other such statements.

Suppose you work with a database that includes the following table populated with data:

CREATE TABLE employees (
  employee_no integer PRIMARY KEY,
  name text,
  department text,
  years_service numeric
);

INSERT INTO employees (employee_no, name, department, years_service)
VALUES
  (1221, 'John Smith', 'Marketing', 5),
  (1222, 'Bette Davis', 'Sales', 3),
  (1223, 'Lucille Ball', 'Operations', 1),
  (1224, 'John Zimmerman', 'Sales', 5);

If you need to find the employees who have been working for the company longer than average, you start by calculating the average years of service using a SELECT statement and average function AVG. Then you use the result of the first query in the second SELECT statement to find the long-serving employees, as shown in the following examples:

SELECT AVG (years_service) FROM employees;

The preceding query returns 3.5000000000000000.

SELECT employee_no, name, years_service FROM employees
  WHERE years_service > 3.5;

The following is the output produced by the preceding example:

employee_no | name             | years_service
------------+------------------+-------------------
1221        | John Smith       | 5
1224        | John Zimmerman   | 5

You can avoid executing two separate queries by using a subquery that is passed the result of the first query. To create such a query, you enclose the second query in brackets and use it as an expression in the WHERE clause, as shown in the following example:

SELECT employee_no, name, years_service FROM employees
  WHERE years_service > (SELECT AVG (years_service) FROM employees);

YSQL executes the subquery first, obtains the result and passes it to the outer query, and finally executes the outer query.

You can use a subquery that is an input of the EXISTS operator which returns true if the subquery returns any rows. The EXISTS operator returns false in cases where the subquery does not return any rows. The EXISTS operator does not access the content of the rows; it only needs to know the number of rows returned by the subquery.

The EXISTS operator has the following syntax:

EXISTS (SELECT 1 FROM some_table WHERE condition);

The following example applies the EXISTS operator to two tables (the employees table and a hypothetical salary table) and shows how to use EXISTS on the employee_no column in a manner similar to an inner join:

SELECT name FROM employees
  WHERE EXISTS (SELECT 1 FROM salary
                  WHERE salary.employee_no = employees.employee_no);

If the salary table existed, the preceding query would have returned no more than one row for each row in the employees table, even though there would have been corresponding rows in the salary table.

Recursive Queries and CTEs

Common Table Expressions (CTEs) allow you to execute recursive, hierarchical, and other types of complex queries in a simplified manner by breaking down these queries into smaller units. A CTE exists only during the query execution and represents a temporary result set that you can reference from another SQL statement.

You can use the following syntax to create a simple CTE:

WITH cte_name (columns) AS (cte_query) statement;

cte_name represents the name of the CTE. columns is an optional list of table columns. cte_query represents a query returning a result set. If columns is not specified, the select list of the cte_query becomes columns of the CTE. statement can be a SELECT, INSERT, UPDATE, or DELETE YSQL statement, and the CTE acts the way a table does in that statement.

Using the fulltime_employees table from Joining Columns, the following example demonstrates how to define a CTE and use it to create a complex query:

WITH cte_fulltime_employees AS (
  SELECT
    ft_name,
      (CASE
         WHEN ft_employee_no < 1222 THEN 'Sales'
         WHEN ft_employee_no > 1223 THEN 'Marketing'
         ELSE 'Operations'
       END) ft_department
  FROM
    fulltime_employees
)
SELECT ft_name, ft_department
FROM cte_fulltime_employees
WHERE ft_department = 'Operations'
ORDER BY ft_name;

The following is the output produced by the preceding example:

 ft_name             | ft_department
---------------------+-------------------
 Bette Davis         | Operations
 Lucille Ball        | Operations

Recursive queries, often used for querying hierarchical data, refer to recursive CTEs.

You can use the following syntax to create a recursive CTE:

WITH RECURSIVE cte_name
  AS(cte_query --- non-recursive
      UNION [ALL]
      cte_query_definition definion  --- recursive
  )
SELECT * FROM cte_name;

cte_name represents the name of the CTE. cte_query represents a non-recursive term which is a CTE query definition creating the base result set of the CTE structure. cte_query_definition represents a recursive term which is one or more CTE query definitions joined with the non-recursive term via the UNION or UNION ALL operator; the CTE name is referenced by the recursive term. The recursion terminates when the previous iteration does not return any rows.

Suppose you work with a database that includes the employees table created and populated as follows:

CREATE TABLE employees (
  employee_no integer PRIMARY KEY,
  name text,
  manager_no integer,
  department text
);

INSERT INTO employees VALUES
  (1221, 'John Smith', NULL, NULL),
  (1222, 'Bette Davis', 1221, 'Sales'),
  (1223, 'Lucille Ball', 1221, 'Operations'),
  (1224, 'John Zimmerman', 1222, 'Sales'),
  (1225, 'Walter Marx', 1222, 'Sales');

The following example demonstrates how to retrieve all employees who report to the manager with manager_no 1222:

WITH RECURSIVE reports AS (
  SELECT employee_no, name, manager_no, department
  FROM employees
  WHERE employee_no = 1222
  UNION
    SELECT
      emp.employee_no, emp.name, emp.manager_no, emp.department
    FROM employees emp
    INNER JOIN reports rep ON rep.employee_no = emp.manager_no
)
SELECT * FROM reports;

In the preceding example, the recursive CTE reports defines a non-recursive and a recursive term, with non-recursive term returning the base result set that includes the employee whose employee_no is 1222. Then the recursive term retrieves the direct reports of the employee whose employee_no is 1222. This is the result of joining the employees table and the reports CTE. The first iteration of the recursive term returns two employees with employee_no 1224 and 1225. The recursive term is executed multiple times. The second iteration uses the preceding result set as the input value but does not return any rows because nobody reports to employees with employee_no 1224 and 1225.

When the final result set is returned, it represents the combination of all result sets in iterations generated by the non-recursive and recursive terms, as demonstrated by the following output:

employee_no | name            | manager_id  | department
------------+-----------------+-------------+--------------
1222        | Bette Davis     | 1221        | Sales
1224        | John Zimmerman  | 1222        | Sales
1225        | Walter Marx     | 1222        | Sales

Another way to execute complex hierarchical queries is to use a tablefunc extension. This extension provides several table functions, such as, for example, normal_rand() that creates values picked using a pseudorandom generator from an ideal normal distribution. For more information and examples, see tablefunc.

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