How Can I Use PostgreSQL's IF Statement for Conditional Database Queries?

Conditional Statements with PostgreSQL IF

PostgreSQL, a powerful open-source database management system, offers various conditional statements to control the flow of execution. One such statement is the IF statement. This article explores how to utilize PostgreSQL's IF statement for conditional operations within database queries.

Issue: Executing Conditional Queries

You request a method to perform a conditional query in Postgres:

IF (select count(*) from orders) > 0<br>THEN<br>  DELETE from orders<br>ELSE <br>  INSERT INTO orders values (1,2,3);<br>

Solution: Using PL/pgSQL and the DO Command

In PostgreSQL, the IF statement is part of the procedural language PL/pgSQL. To execute conditional queries, one must employ the DO command to create a function or execute an ad-hoc statement:

DO<br>$do$<br>BEGIN<br>   IF EXISTS (SELECT FROM orders) THEN</p>
<pre class="brush:php;toolbar:false">  DELETE FROM orders;

ELSE

  INSERT INTO orders VALUES (1,2,3);

END IF;
END
$do$

Here's a breakdown of the code:

• Parentheses (brackets) are necessary around the sub-select: (SELECT count(*) FROM orders).
• A semicolon (;) separates each statement in PL/pgSQL, except for the final END.
• The IF statement must conclude with END IF;.
• Instead of a sub-select, consider using IF EXISTS (SELECT FROM orders) for improved performance.

An Alternative Approach

To simplify the query, you can omit the additional SELECT statement and write the following:

DO<br>$do$<br>BEGIN<br>   DELETE FROM orders;<br>   IF NOT FOUND THEN</p>
<pre class="brush:php;toolbar:false">  INSERT INTO orders VALUES (1,2,3);

END IF;
END
$do$

While this method is efficient, concurrent transactions writing to the same table may cause interference. To mitigate this, it's advisable to write-lock the table within the transaction.

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