How Can I Implement If-Then-Else Logic in SQL Queries?

Can SQL Handle If-Then-Else Logic?

You need to retrieve data from a table based on priority, and you're unsure how to approach this in SQL. Let's delve into this query and explore the options for handling conditional logic.

Handling Conditional Logic in SQL

Contrary to traditional programming languages, SQL does not possess if-then-else statements. However, there are several approaches to mimic conditional logic:

Using CASE Statements

CASE statements provide a flexible way to handle conditional logic. You can specify multiple conditions and return different results based on each condition. Here's an example:

SELECT CASE
  WHEN (SELECT COUNT(*) FROM table1 WHERE project = 1) > 0
  THEN (SELECT product, price FROM table1 WHERE project = 1)
  WHEN (SELECT COUNT(*) FROM table1 WHERE customer = 2) > 0
  THEN (SELECT product, price FROM table1 WHERE customer = 2)
  WHEN (SELECT COUNT(*) FROM table1 WHERE company = 3) > 0
  THEN (SELECT product, price FROM table1 WHERE company = 3)
  ELSE 'No price found'
END;

Using IF-THEN-ELSE Statement (Procedural SQL)

In procedural SQL, you can use the IF-THEN-ELSE statement to check conditions and execute different query blocks based on the outcome. Keep in mind that MS SQL does not support IF-THEN-ELSE directly. However, you can simulate it using a CASE statement or the sp_if statement.

Using UNION Operators

UNION operators can be used to combine the results of multiple queries into a single result set. This approach is useful if you want to prioritize one query over others, as the earlier queries will be checked first.

SELECT product, price FROM table1 WHERE project = 1
UNION
SELECT product, price FROM table1 WHERE customer = 2
UNION
SELECT product, price FROM table1 WHERE company = 3;

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