SQL where子句性能问题中的选择计数

Question

由于where子句中的select count（1）语句，我具有以下可怕的SQL查询。 谁能建议一种可以加快速度的方法？ 我的想法是，我只希望返回找到一张发票​​的行。

SELECT people.name, people.address 
FROM people 
WHERE ((SELECT COUNT(1) FROM invoices WHERE invoices.pid = people.id)=1)

Answer 1

• COUNT（1）是迷信
• 您所拥有的是每行人员的数量=光标/循环之类的动作

因此，尝试这样的JOIN

SELECT people.name, people.address 
FROM
   people 
   JOIN
   invoices ON invoices.pid = people.id
GROUP BY
   people.name, people.address 
HAVING
   COUNT(*) = 1

我也希望您至少在invoices.pid和people.pid上具有索引，名称，地址

Answer 2

使用JOIN ：

SELECT people.name, people.address
FROM people
JOIN invoices ON invoices.pid = people.id
GROUP BY people.name, people.address
HAVING Count(*) = 1

Answer 3

我认为，加入表格在实践和性能上可能会更好。

SELECT people.name, people.address 
FROM people INNER JOIN invoices ON invoices.pid = people.id

由于正在编辑OP而进行编辑：您只需要那些只拥有一张发票的人吗？ 如果是这样，则无视此问题，并查看其他答案之一。