How Can I Efficiently Execute a Second SQL SELECT Query Only if the First Returns No Rows?

Executing a Conditional Second SELECT Query Based on the Results of the First

Enhancing the efficiency of PHP scripts can involve leveraging database operations. One optimization technique involves executing a different SELECT query only when the first SELECT query returns no rows or a specific condition is met.

For example, consider the following queries:

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

To conditionally execute the second query only if the first query returns an empty set, the following approach can be used:

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 approach involves running each query twice, once for counting and once for retrieving data. A more efficient solution is to utilize UNION ALL 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 from the proxies table where A='B' if they exist. If they don't exist, it fetches rows with A='C'. This approach significantly reduces the number of database calls, enhancing script performance.

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