How Can I Implement If-Then-Else Logic Using SQL's CASE Statement?

If-Then-Else Logic in SQL

SQL offers the ability to perform conditional logic using the CASE statement. While it may not provide direct syntax for if-then-else constructs, it enables you to define a series of conditions and corresponding actions.

Problem:

Suppose you want to select data from a table based on specific priorities:

• If project has a record, return it.
• If customer has a record, return it.
• If company has a record, return it.

Solution Using CASE:

SELECT product, price
FROM table1
WHERE CASE
  WHEN EXISTS (SELECT 1 FROM table1 WHERE project = 1) THEN 1
  WHEN EXISTS (SELECT 1 FROM table1 WHERE customer = 2) THEN 2
  WHEN EXISTS (SELECT 1 FROM table1 WHERE company = 3) THEN 3
  ELSE NULL
END IS NOT NULL;

This query evaluates each condition sequentially. If project has a record, it returns the matching row; otherwise, it checks for customer and then company. Only rows that satisfy any of the conditions will be selected.

Example:

The following table contains some sample data:

product	price	project	customer	company
A	10	1	NULL	NULL
B	20	NULL	2	NULL
C	30	NULL	NULL	3

Executing the query will return the following results:

product	price
A	10

Since there is a row with project = 1, the other conditions are ignored, and the row with product "A" is selected.

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