如何在不使用多个子查询的情况下使用多列选择多行

【问题标题】：How do I select multiple rows using multiple columns without using multiple subqueries如何在不使用多个子查询的情况下使用多列选择多行
【发布时间】：2020-09-01 12:29:06
【问题描述】：

我有以下疑问：

SELECT name, modelName, color, rarity, slot, power, chapter
FROM items
WHERE items.id = (SELECT head FROM characterdata WHERE steamid = ?)
OR items.id = (SELECT leftHand FROM characterdata WHERE steamid = ?)
OR items.id = (SELECT rightHand FROM characterdata WHERE steamid = ?)
OR items.id = (SELECT feet FROM characterdata WHERE steamid = ?);

它必须执行四次完全相同的查询，这似乎是不好的做法。我怎样才能重写它，让它只使用两个查询，也就是只需要获取一次字符数据？

【问题讨论】：

标签： mysql sql optimization query-optimization

【解决方案1】：

你可以使用exists：

SELECT name, modelName, color, rarity, slot, power, chapter
FROM items i
WHERE EXISTS (SELECT 1
              FROM characterdata cd
              WHERE cd.steamid = ? AND
                    i.id IN (cd.head, cd.leftHand, cd.rightHand, cd.feet)
             );

这假定? 始终具有相同的值。

如果不是，你可以这样表达：

SELECT name, modelName, color, rarity, slot, power, chapter
FROM items i
WHERE EXISTS (SELECT 1
              FROM characterdata cd
              WHERE (cd.steamid = ? AND i.id = cd.head) OR
                    (cd.steamid = ? AND i.id = cd.leftHand) OR
                    (cd.steamid = ? AND i.id = cd.rightHand) OR
                    (cd.steamid = ? AND i.id = cd.feet)
             );

【讨论】：