Question

这两个SQL语句返回相同的结果，但第一个比第二个慢得多：

SELECT leading.email, kstatus.name, contacts.status 
FROM clients 
JOIN clients_leading ON clients.id_client = clients_leading.id_client 
JOIN leading ON clients_leading.id_leading = leading.id_leading 
JOIN contacts ON contacts.id_k_p = clients_leading.id_group 
JOIN kstatus on contacts.status = kstatus.id_kstatus 
WHERE (clients.email = 'some_email' OR clients.email1 = 'some_email') 
ORDER BY contacts.date DESC;



SELECT leading.email, kstatus.name, contacts.status
FROM (
     SELECT * 
     FROM clients 
     WHERE (clients.email = 'some_email' OR clients.email1 = 'some_email')
     ) 
AS clients  
JOIN clients_leading ON clients.id_client = clients_leading.id_client 
JOIN leading ON clients_leading.id_leading = leading.id_leading  
JOIN contacts ON contacts.id_k_p = clients_leading.id_group  
JOIN kstatus on contacts.status = kstatus.id_kstatus 
ORDER BY contacts.date DESC;

但我想知道为什么会这样？看起来在firt语句中首先完成连接然后应用WHERE子句，而第二个恰好相反。但它在所有数据库引擎上的行为方式是否相同（我在MySQL上测试过它）？

我期待数据库引擎可以优化查询，例如fors one和firs应用WHERE子句然后进行连接。

Answer 1

这可能有很多不同的原因（键控等），但您可以查看explain mysql命令以查看语句的执行方式。如果你可以运行它，如果它仍然是一个神秘的帖子。

Answer 2

你总是可以用嵌套查询替换连接...它总是更快但很麻烦......

SQL连接vs嵌套查询

2 个答案: