How Can I Optimize MySQL Queries by Combining SELECT Statements Based on Row Counts?

Combining SELECT Queries Based on Row Count

In PHP, optimizing database queries can improve script performance. One technique is pushing logic into MySQL by conditionally executing a second SELECT query if the first returns zero rows.

Consider the following example:

SELECT * FROM proxies WHERE (A='B') || SELECT * FROM proxies WHERE (A='C')

This code runs both SELECT queries regardless of the first query's result.

A better approach is to use MySQL's conditional execution feature, as seen in this code:

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 still executes the count queries twice.

A more efficient solution is to utilize UNION ALL in conjunction with EXISTS:

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 retrieves rows with A='B' if they exist, and if not, it retrieves rows with A='C.' By using the EXISTS condition, the query only retrieves rows for the first specified condition (A='B') if they exist, resulting in a single execution of the count query.

SQL Fiddle Demo: https://www.sqlfiddle.com/#!9/3de314

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