How Can I Efficiently Execute a Second SELECT Query in MySQL Only if the First Query Returns Zero Rows?

Conditional SELECT Queries: Executing Alternative Queries Based on Row Count

To optimize your PHP script, you're seeking a method to dynamically execute a second SELECT query only when the first query returns zero rows. Here's how you can achieve this in MySQL:

One approach is to use nested IF statements to compare the count of rows from the first query:

IF (SELECT COUNT(*) FROM proxies WHERE A='B') > 0
  THEN SELECT * FROM proxies WHERE A='B'
ELSEIF (SELECT COUNT(*) FROM proxies WHERE A='C') > 0
  THEN SELECT * FROM proxies WHERE A='C'
END IF

However, this method executes each query twice for each condition. For better optimization, consider using UNION ALL with the EXISTS operator:

SELECT *
FROM proxies
WHERE A='B'
UNION ALL
SELECT *
FROM proxies
WHERE A='C' AND NOT EXISTS (
  SELECT 1
  FROM proxies
  WHERE A='B'
)

This query will first look for rows with A='B' and return them. If no rows with A='B' are found, it will then fetch rows with A='C', ensuring that only one query is executed.

An SQL Fiddle demo can be found here: [Link]

This approach efficiently executes the second query only when the first query returns zero rows, providing improved performance for your PHP script.

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