How Can I Implement Conditional Logic (like if-then-else) in SQL Queries?

Implementing Conditional Logic with SQL

Question:

How can conditional statements, similar to the if-then-else logic, be implemented in SQL to retrieve data based on specific priorities?

Answer:

While SQL does not offer direct if-then-else statements, it provides alternative constructs to achieve conditional logic.

Using IF-THEN-ELSE Statements:

The snippet below demonstrates how to emulate if-then-else logic using the IF keyword in MS SQL:

IF ((SELECT COUNT(*) FROM table1 WHERE project = 1) > 0) 
    SELECT product, price FROM table1 WHERE project = 1
ELSE IF ((SELECT COUNT(*) FROM table1 WHERE project = 2) > 0) 
    SELECT product, price FROM table1 WHERE project = 2
ELSE IF ((SELECT COUNT(*) FROM table1 WHERE project = 3) > 0)
    SELECT product, price FROM table1 WHERE project = 3

Explanation:

• This query first checks if any rows exist in table1 where project equals 1. If so, it selects and returns data from those rows.
• If no rows are found, it checks if any rows exist where project equals 2 and returns data from those rows.
• This process continues until data is retrieved or all conditions are exhausted.

Alternative Approaches:

• CASE Statements: CASE statements can be used to evaluate multiple conditions and return different values based on the result.
• UNION: UNION statements can combine multiple queries and return distinct rows from each query, allowing for conditional data selection.

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