结合三个SQL查询

Question

我有以下三个SQL查询。我想将它们合并成一个是可能的，要么使用当前的方法，要么使用其他方法，如果它会更有效。

注意：我需要知道所有三个WHERE子句的不同计数值。

查询1：

SElECT COUNT(*) AS imgCount FROM (
    SELECT imgHeight, imgWidth, imgId AS primaryId FROM primary_images 
    UNION ALL 
    SELECT imgHeight, imgWidth, primaryId FROM secondary_images 
) AS union_table 
WHERE primaryId = $imgId AND imgWidth = $maxImageWidth AND imgHeight = $maxImageHeight; 

QUERY 2：

SElECT COUNT(*) AS imgCount FROM (
    SELECT imgHeight, imgWidth, imgId AS primaryId FROM primary_images 
    UNION ALL 
    SELECT imgHeight, imgWidth, primaryId FROM secondary_images 
) AS union_table 
WHERE primaryId = $imgId AND imgWidth = $maxImageWidth AND imgHeight != $maxImageHeight; 

QUERY 3：

SElECT COUNT(*) AS imgCount FROM (
    SELECT imgHeight, imgWidth, imgId AS primaryId FROM primary_images 
    UNION ALL 
    SELECT imgHeight, imgWidth, primaryId FROM secondary_images 
) AS union_table 
WHERE primaryId = $imgId AND imgWidth != $maxImageWidth AND imgHeight = $maxImageHeight; 

---

另外，我的数据库是MySQL。我听说使用!=作为“不等于”在所有情况下都不起作用，但是<>更好。在我的情况下，应该!=罚款？

Answer 1

这可能适合你。

SElECT COUNT(*) AS imgCount FROM (
    SELECT imgHeight, imgWidth, imgId AS primaryId FROM primary_images 
    UNION ALL 
    SELECT imgHeight, imgWidth, primaryId FROM secondary_images 
) AS union_table 
WHERE primaryId = $imgId 
AND (imgWidth != $maxImageWidth AND imgHeight = $maxImageHeight 
OR imgWidth = $maxImageWidth AND imgHeight != $maxImageHeight 
OR imgWidth = $maxImageWidth AND imgHeight = $maxImageHeight); 

我也相信这是等同的，虽然我不确定。

SElECT COUNT(*) AS imgCount FROM (
    SELECT imgHeight, imgWidth, imgId AS primaryId FROM primary_images 
    UNION ALL 
    SELECT imgHeight, imgWidth, primaryId FROM secondary_images 
) AS union_table 
WHERE primaryId = $imgId 
AND (imgWidth = $maxImageWidth OR imgHeight = $maxImageHeight); 

根据请求，这里是一个返回3行的单个查询。

SELECT 'ALL EQUAL' AS COL1,COUNT(*) AS imgCount FROM (
    SELECT imgHeight, imgWidth, imgId AS primaryId FROM primary_images 
    UNION ALL 
    SELECT imgHeight, imgWidth, primaryId FROM secondary_images 
) AS union_table 
WHERE primaryId = $imgId AND imgWidth = $maxImageWidth AND imgHeight = $maxImageHeight 
UNION ALL 
SELECT 'WIDTH EQUAL' AS COL1,COUNT(*) AS imgCount FROM (
    SELECT imgHeight, imgWidth, imgId AS primaryId FROM primary_images 
    UNION ALL 
    SELECT imgHeight, imgWidth, primaryId FROM secondary_images 
) AS union_table 
WHERE primaryId = $imgId AND imgWidth = $maxImageWidth AND imgHeight != $maxImageHeight; 
UNION ALL 
SELECT 'HEIGHT EQUAL' AS COL1,COUNT(*) AS imgCount FROM (
    SELECT imgHeight, imgWidth, imgId AS primaryId FROM primary_images 
    UNION ALL 
    SELECT imgHeight, imgWidth, primaryId FROM secondary_images 
) AS union_table 
WHERE primaryId = $imgId AND imgWidth != $maxImageWidth AND imgHeight = $maxImageHeight; 

Answer 2

我认为需要一个CASE和GROUP BY：

SELECT CASE 
     WHEN imgWidth = $maxImageWidth AND imgHeight = $maxImageHeight 
     THEN 0 
     WHEN imgWidth != $maxImageWidth AND imgHeight = $maxImageHeight 
     THEN 1 
     WHEN imgWidth = $maxImageWidth AND imgHeight != $maxImageHeight 
     THEN 2 
     END AS count_type, 
     COUNT(*) AS imgCount 
    FROM (SELECT imgHeight, imgWidth, imgId AS primaryId FROM primary_images 
     UNION ALL 
     SELECT imgHeight, imgWidth, primaryId FROM secondary_images 
     ) AS union_table 
WHERE primaryId = $imgId 
    AND (imgWidth = $maxImageWidth OR imgHeight = $maxImageHeight) 
GROUP BY count_type; 

这至少是诱人推WHERE子句下入UNION子查询，但优化应该做的反正。通过学习EXPLAIN输出来检查它是否确实可以做到这一点。如果优化不会自动做谓语下推，你会写：

SELECT CASE 
     WHEN imgWidth = $maxImageWidth AND imgHeight = $maxImageHeight 
     THEN 0 
     WHEN imgWidth != $maxImageWidth AND imgHeight = $maxImageHeight 
     THEN 1 
     WHEN imgWidth = $maxImageWidth AND imgHeight != $maxImageHeight 
     THEN 2 
     END AS count_type, 
     COUNT(*) AS imgCount 
    FROM (SELECT imgHeight, imgWidth, imgId AS primaryId FROM primary_images 
     WHERE primaryId = $imgId -- or: imgId = $imgId 
      AND (imgWidth = $maxImageWidth OR imgHeight = $maxImageHeight) 
     UNION ALL 
     SELECT imgHeight, imgWidth, primaryId FROM secondary_images 
     WHERE primaryId = $imgId 
      AND (imgWidth = $maxImageWidth OR imgHeight = $maxImageHeight) 
     ) AS union_table 
GROUP BY count_type; 

书面这种解决方案的缺点是$maxImageWidth和$maxImageHeight变量每次使用（和$imgId一次）的4倍。如果你准备的语句如图所示（在表示SQL的字符串中嵌入值），这并不算太坏。如果您使用的占位符，你需要的占位符，如果可能的命名方案（:img_ht，:img_wd，:img_id（或至少:1，:2，:3）代替?占位符），所以你只能在EXECUTE语句一次通过每个值。不过，这种能力取决于你的数据库管理系统; MySQL是许多不支持这种功能的DBMS之一。

Answer 3

你在primary_images和secondary_images表中的primaryId上有索引吗？如果是的话，那么你应该推动里面的where子句，以便可以使用索引。

SElECT COUNT(*) AS imgCount FROM (
    SELECT imgHeight, imgWidth, imgId AS primaryId FROM primary_images WHERE imgId = $imgId AND imgWidth != $maxImageWidth AND imgHeight = $maxImageHeight 
    UNION ALL 
    SELECT imgHeight, imgWidth, primaryId FROM secondary_images WHERE primaryId = $imgId AND imgWidth != $maxImageWidth AND imgHeight = $maxImageHeight 
) AS union_table 

，否则这将是一个全表扫描找到imgId为union_table是由MySQL创建临时表中，没有任何指标。