Home Database Mysql Tutorial mysql 判断记录是否存在方法比较

mysql 判断记录是否存在方法比较

Jun 07, 2016 pm 06:05 PM
mysql

把数据写入到数据库的时,常常会碰到先要检测要插入的记录是否存在,然后决定是否要写入。

我这里总结了判断记录是否存在的常用方法:

sql语句:select count(*) from tablename;

然后读取count(*)的值判断记录是否存在。对于这种方法性能上有些浪费,我们只是想判断记录记录是否存在,没有必要全部都查出来。

以下这个方法是我推荐的。

sql语句:select 1 from tablename where col = col limit 1;

然后读取语句执行所影响的行数。

当然这里limit 1很重要。这要mysql找到一条记录后就不会在往下找了。这里执行所影响的行数不是0就是1,性能提高了不少。

如果你用的是PDO,可以用rowCount(),很容易就都到执行所影响的行数。

这里还有人可能会去读取sql语句查询到的记录,然后判断记录是否存在,从而判断记录是否存在。这个方法虽然可行,但对于我们的要求来说,还是有些浪费,我们不需要查询到的记录,所有性能上会有损失。这里不推荐。
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