计算表中未引用的行数

问题描述

我有一个表addresses和许多表，其中有一个引用地址的address_id列（都具有外键约束）。我想找到其他表中未引用的地址数。

我尝试了以下查询，但是速度很慢：

SELECT COUNT(*)
FROM addresses A
LEFT JOIN customers C ON C.address_id = A.id
LEFT JOIN agreements AG ON AG.address_id = A.id
LEFT JOIN products P ON P.address_id = A.id
LEFT JOIN letters L ON L.address_id = A.id
WHERE C.id IS NULL
AND AG.id IS NULL
AND P.id IS NULL
AND L.id IS NULL

有什么方法可以做到这一点，而不会使查询永远占用？

解决方法

我首先用not exists重写它：

select count(*)
from addresses a
where 
        not exists (select 1 from customers c  where c.address_id = a.id)
    and not exists (select 1 from agreements g where g.address_id = a.id)
    and not exists (select 1 from products p   where p.address_id = a.id)
    and not exists (select 1 from letters l    where l.address_id = a.id)

然后，请确保具有以下索引以加快查询速度：

customers(address_id)
agreements(address_id)
products(address_id)
letters(address_id)

如果您已正确地将address_id列定义为address(id)的外键，则这些索引已经存在。