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HomeDatabaseMysql TutorialMySQL大表中重复字段的高效率查询方法

MySQL大表重复字段应该如何查询到呢?这是很多人都遇到的问题,下面就教您一个MySQL大表重复字段的查询方法,供您参考

MySQL大表重复字段应该如何查询到呢?这是很多人都遇到的问题,下面就教您一个MySQL大表重复字段的查询方法,供您参考。

数据库中有个大表,需要查找其中的名字有重复的记录id,以便比较。
如果仅仅是查找数据库中name不重复的字段,很容易
代码如下:
SELECT min(`id`),`name`
FROM `table`
GROUP BY `name`;

但是这样并不能得到说有重复字段的id值。(只得到了最小的一个id值)
查询哪些字段是重复的也容易
代码如下:
SELECT `name`,count(`name`) as count
FROM `table`
GROUP BY `name` HAVING count(`name`) >1
ORDER BY count DESC;

但是要一次查询到重复字段的id值,就必须使用子查询了,于是使用下面的语句来实现MySQL大表重复字段查询。
代码如下:
SELECT `id`,`name`
FROM `table`
WHERE `name` in (
SELECT `name`
FROM `table`
GROUP BY `name` HAVING count(`name`) >1
);

但是这条语句在mysql中效率太差,感觉mysql并没有为子查询生成临时表。
于是使用先建立临时表
代码如下:
create table `tmptable` as (
SELECT `name`
FROM `table`
GROUP BY `name` HAVING count(`name`) >1
);

然后使用多表连接查询
代码如下:
SELECT a.`id`, a.`name`
FROM `table` a, `tmptable` t
WHERE a.`name` = t.`name`;

结果这次结果很快就出来了。

用 distinct去重复
代码如下:
SELECT distinct a.`id`, a.`name`
FROM `table` a, `tmptable` t
WHERE a.`name` = t.`name`;

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