How Can I Implement If-Then-Else Logic for Conditional Data Retrieval in SQL?

Conditional Execution in SQL: If-Then-Else Logic

Conditional logic is essential in programming for making decisions based on specific conditions. In SQL, you might need to retrieve data from tables based on a sequence of priorities. To achieve this, you can employ the following approaches:

Using CASE Statements:

CASE statements allow you to evaluate an expression and perform different actions based on the result. You can use them similarly to if-else statements to check for multiple conditions and execute corresponding queries:

SELECT
  product,
  price
FROM table1
WHERE project = 1
UNION ALL
CASE
  WHEN (SELECT COUNT(*) FROM table1 WHERE customer = 2) > 0
  THEN (
    SELECT
      product,
      price
    FROM table1
    WHERE customer = 2
  )
  ELSE (
    SELECT
      product,
      price
    FROM table1
    WHERE company = 3
  )
END;

Using IF-ELSE Statements in SQL Server:

In Microsoft SQL Server, you can use IF-ELSE statements directly within your queries to achieve similar functionality:

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

Both approaches enable you to execute conditional queries and retrieve data based on specific priorities. Choose the method that best suits your specific requirements.

The above is the detailed content of How Can I Implement If-Then-Else Logic for Conditional Data Retrieval in SQL?. For more information, please follow other related articles on the PHP Chinese website!