mysql - PHP SQL IF NOT EXISTS 将不起作用

Question

我有一个关于 SQL (php) 的问题，我想在我的表中插入数据。但我想使用 IF NOT EXIST 值。我试过的:

INSERT INTO vrienden (id, userid, vriendmetid, accepted) VALUES (null, '1', '20', '0') WHERE NOT EXISTS (SELECT * FROM vrienden WHERE userid='1' AND vriendmetid='20')

我不确定出了什么问题，因为我收到以下错误:

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE NOT EXISTS (SELECT * FROM vrienden WHERE userid='1' AND vriendmetid='20')' at line 1

谢谢。

Answer 1

你想要 insert 。 . .选择，而不是插入。 . .值(value)观:

INSERT INTO vrienden (id, userid, vriendmetid, accepted) 
     SELECT x.*
     FROM (select null as id, '1' as userid, '20' as vriendmetid, '0' as accepted) x
     WHERE NOT EXISTS (SELECT 1 FROM vrienden v WHERE v.userid = x.userid AND v.vriendmetid = x.vriendmetid);

但是，您可能不应该在 INSERT 中执行此操作。相反，创建一个唯一的索引/约束:

create unique index unq_vrienden_userid_vriendmetid on vrienden(userid, vriendmetid);

这样，数据库将确保列的唯一性，因此您的应用程序不必这样做。