mysql - 结合多个 MySQL UNION 查询进行优化

Question

我必须组合这些多个 UNION 查询...

SELECT 'Mercedes' AS marke, COUNT(autos.id) FROM autos WHERE a1=1 
UNION 
SELECT 'BMW' AS marke, COUNT(autos.id) FROM autos WHERE a2=1 
UNION 
SELECT 'Ford' AS marke, COUNT(autos.id) FROM autos WHERE a3=1

我怎样才能只用一个查询来做到这一点？

Answer 1

如果您希望摆脱 UNION，您可以使用带有 CASE 表达式的聚合查询来解码 marke。

SELECT
    CASE
        WHEN a1 = 1 THEN 'Mercedes'
        WHEN a2 = 1 THEN 'BMW'
        WHEN a3 = 1 THEN 'Ford'
    END AS marke,
    COUNT(autos.id)
FROM autos
WHERE a1 = 1 OR a2 = 1 OR a3 = 1
GROUP BY a1, a2, a3