Embedded SQL

You can embed and execute SQL statements and expressions from within stored procedures.

Assignment

To save the value of an expression or returned value, you can assign it to a variable:

variable_name := expression;
variable_name := statement;

For example, this procedure assigns 3 into i and 'message' into v.

=> CREATE PROCEDURE performless_assignment() LANGUAGE PLvSQL AS $$
DECLARE
    i int;
    v varchar;
BEGIN
    i := SELECT 3;
    v := 'message';
END;
$$;

This type of assignment will fail if the query returns no rows or more than one row. For returns of multiple rows, use LIMIT or truncating assignment:

=> SELECT * FROM t1;
 b
---
 t
 f
 f
(3 rows)

        
=> CREATE PROCEDURE more_than_one_row() LANGUAGE PLvSQL as $$
DECLARE
    x boolean;
BEGIN
    x := SELECT * FROM t1;
END;
$$;
CREATE PROCEDURE

=> CALL more_than_one_row();
ERROR 10332:  Query returned multiple rows where 1 was expected

Truncating Assignment

Truncating assignment stores in a variable the first row returned by a query. Row order is nondeterministic unless you specify an ORDER BY clause:

variable_name <- expression;
variable_name <- statement;

The following procedure takes the first row of the results returned by the specified query and assigns it to x:

=> CREATE PROCEDURE truncating_assignment() LANGUAGE PLvSQL AS $$
DECLARE
    x boolean;
BEGIN
    x <- SELECT * FROM t1 ORDER BY b DESC; -- x is now assigned the first row returned by the SELECT query
END;
$$;

PERFORM

The PERFORM keyword runs a SQL statement or expression and discards the returned result.

PERFORM statement;
PERFORM expression;

For example, this procedure inserts a value into a table. INSERT returns the number of rows inserted, so you must pair it with PERFORM.

=> DO $$
BEGIN
    PERFORM INSERT INTO coordinates VALUES(1,2,3);
END;
$$;

If a SQL statement has no return value or you don't assign the return value to a variable, you must use PERFORM.

EXECUTE

EXECUTE allows you to dynamically construct a SQL query during execution:

EXECUTE command_expression [ USING expression [, ... ] ];

command_expression is a SQL expression that can reference PL/vSQL variables and evaluates to a string literal. The string literal is executed as a SQL statement, and $1, $2, ... are substituted with the corresponding expressions.

Constructing your query with PL/vSQL variables can be dangerous and expose your system to SQL injection, so wrap them with QUOTE_IDENT, QUOTE_LITERAL, and QUOTE_NULLABLE.

The following procedure constructs a query with a WHERE clause:

DO $$
BEGIN
    EXECUTE 'SELECT * FROM t1 WHERE x = $1' USING 10; -- becomes WHERE x = 10
END;
$$;

The following procedure creates a user with a password from the username and password arguments. Because the constructed CREATE USER statement uses variables, use the functions QUOTE_IDENT and QUOTE_LITERAL, concatenating them with ||.

=> CREATE PROCEDURE create_user(username varchar, password varchar) LANGUAGE PLvSQL AS $$
BEGIN
    EXECUTE 'CREATE USER ' || QUOTE_IDENT(username) || ' IDENTIFIED BY ' || QUOTE_LITERAL(password);
END;
$$;

EXECUTE is a SQL statement, so you can assign it to a variable or pair it with PERFORM:

variable_name:= EXECUTE command_expression;
PERFORM EXECUTE command_expression;

FOUND (special variable)

The special boolean variable FOUND is initialized as false and assigned true or false based on whether:

A statement (but not expression) returns results with non-zero number of rows, or
A FOR loop iterates at least once

You can use FOUND to distinguish between a NULL and 0-row return.

Special variables exist between the scope of a procedure's argument and the outermost block of its definition. This means that:

Special variables shadow procedure arguments
Variables declared in the body of the stored procedure will shadow the special variable

The following procedure demonstrates how FOUND changes. Before the SELECT statement, FOUND is false; after the SELECT statement, FOUND is true.

=> DO $$
BEGIN
    RAISE NOTICE 'Before SELECT, FOUND = %', FOUND;
    PERFORM SELECT 1; -- SELECT returns 1
    RAISE NOTICE 'After SELECT, FOUND = %', FOUND;
END;
$$;

NOTICE 2005:  Before SELECT, FOUND = f
NOTICE 2005:  After SELECT, FOUND = t

Similarly, UPDATE, DELETE, and INSERT return the number of rows affected. In the next example, UPDATE doesn't change any rows, but returns the value 0 to indicate that no rows were affected, so FOUND is set to true:

=> SELECT * t1;
  a  |  b
-----+-----
 100 | abc
(1 row)

DO $$
BEGIN
    PERFORM UPDATE t1 SET a=200 WHERE b='efg'; -- no rows affected since b doesn't contain 'efg'
    RAISE INFO 'FOUND = %', FOUND;
END;
$$;

INFO 2005:  FOUND = t

FOUND starts as false and is set to true if the loop iterates at least once: