MySQL 5.5 在 WHERE 子句中使用 CASE 语句时不使用索引

Question

I have a table of vehicles with a VARCHAR(16) PRIMARY key on a field called "stockno". The table has over 9K records. I am trying to to develop an api that returns a list of vehicles if a stockno arg is null otherwise it returns the specified stockno. However when I run an EXPLAIN on the query below the index is not use.

SET @stockno = 'abc';
SELECT stockno FROM vehicles WHERE CASE WHEN @stockno IS NOT NULL THEN vehicles.stockno = @stockno ELSE 1 = 1 END

I have tried: SELECT stockno FROM vehicles WHERE CASE WHEN @stockno IS NOT NULL THEN vehicles.stockno = CONVERT(@stockno USING latin1) ELSE 1 = 1 END

Also when I run SELECT stockno FROM vehicles WHERE stockno = 'abcdef' the index is used.

Edit This statement will eventually be used as a view.

Answer 1

You can change the query to use UNION ALL

SET @stockno = 'abc';
SELECT stockno
FROM vehicles
WHERE @stockno IS NOT NULL
AND vehicles.stockno = @stockno
UNION ALL
SELECT stockno
FROM vehicles
WHERE @stockno IS NULL;

Answer 2

在查看 SQL 后，我认为通过 API 创建动态 SQL 字符串是最简单的方法。