How Do I Use IF-ELSE Statements in PostgreSQL's PL/pgSQL?

Understanding the PostgreSQL IF Statement

In PostgreSQL, procedural elements such as conditional statements require the use of the PL/pgSQL language. To implement an IF-ELSE statement in PostgreSQL, follow these steps:

Creating the IF Statement

1. Begin the statement with DO and enclose the procedural logic within a dollar-quoted code block ($do$):

DO
$do$

2. Use the IF keyword to initiate the conditional statement:

IF EXISTS (SELECT FROM orders) THEN

3. Specify the conditional expression within parentheses. Here, EXISTS checks if rows exist in the orders table:

IF (SELECT count(*) > 0 FROM orders) ...

4. Use THEN and ELSE to define the actions to execute for true and false conditions:

    IF ... THEN
        DELETE FROM orders;
    ELSE
        INSERT INTO orders VALUES (1,2,3);
    END IF;

End the Statement

1. Close the IF statement with END IF;.
2. Terminate the dollar-quoted code block and close the DO statement:

END
$do$

Example

The following example demonstrates how to implement the IF-ELSE statement according to the above steps:

DO
$do$
BEGIN
   IF EXISTS (SELECT FROM orders) THEN
      DELETE FROM orders;
   ELSE
      INSERT INTO orders VALUES (1,2,3);
   END IF;
END
$do$

Additional Notes

• Semicolons (;) are required at the end of each PL/pgSQL statement except for the final END.
• PostgreSQL uses the EXISTS operator for row existence checks, which is more efficient than a sub-select.
• Consider write-locking the table before executing the statement to prevent concurrent write conflicts.

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