mysql删除重复记录，保存Id最小的一条

admin

方法1：
; b& Y" F, K4 _1 o3 }! X/ j6 }9 q: k/ s1、创建一个临时表，选取需要的数据。
2、清空原表。
# @9 Z# H1 a& {3 [3、临时表数据导入到原表。
4、删除临时表。
mysql> select * from student;
  N# R4 G5 D- S% {- s  b3 N; H7 A+----+------+
| ID | NAME |
0 p* s. g  s4 @  Y2 L+----+------+
& w1 y) g9 _9 Z8 b/ ^3 f| 11 | aa |
| 12 | aa |
| 13 | bb |
| 14 | bb |
4 p% G! y. L/ g' x1 J4 N| 15 | bb |
| 16 | cc |
5 g$ |  ^; M9 h  |0 j4 u+----+------+
6 rows in set

mysql> create temporary table temp as select min(id),name from student group by name;
( |7 J3 p# j$ v, mQuery OK, 3 rows affected
Records: 3 Duplicates: 0 Warnings: 0

mysql> truncate table student;
% m$ N) ]) P% @$ e2 n! A) j) WQuery OK, 0 rows affected

mysql> insert into student select * from temp;
Query OK, 3 rows affected
Records: 3 Duplicates: 0 Warnings: 0

mysql> select * from student;
+----+------+
| ID | NAME |
  S2 Y( \& u5 c. b( H+----+------+
6 o% R+ ?& p/ o1 a| 11 | aa |
| 13 | bb |
| 16 | cc |
+ r, M4 E* Y( H/ h& e8 _+----+------+
3 rows in set

mysql> drop temporary table temp;
* ^) N2 I( ~' Q% x  ?: l) {( O2 HQuery OK, 0 rows affected
这个方法，显然存在效率问题。

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方法2：按name分组，把最小的id保存到临时表，删除id不在最小id集合的记录，如下：
mysql> create temporary table temp as select min(id) as MINID from student group by name;
5 g& {) p. t$ U2 @8 jQuery OK, 3 rows affected
Records: 3 Duplicates: 0 Warnings: 0

mysql> delete from student where id not in (select minid from temp);
, G% N  R) ~5 C9 U3 ]8 G" ]$ lQuery OK, 3 rows affected

mysql> select * from student;
+----+------+
- ^* m- H5 Z7 R# u| ID | NAME |
4 z! m( Z* S4 h: N- r) U# ^" M+----+------+
| 11 | aa |
5 c$ \8 R' c' O: G4 R| 13 | bb |
' @8 ?: I+ h2 w+ m6 ^# v" d| 16 | cc |
+----+------+
3 rows in set

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方法3：直接在原表上操作，容易想到的sql语句如下：

mysql> delete from student where id not in (select min(id) from student group by name);
% q5 d/ N2 C5 C$ s: q& Z执行报错：1093 - You can't specify target table 'student' for update in FROM clause
1 l  @( W3 V6 q- n$ p原因是：更新数据时使用了查询，而查询的数据又做了更新的条件，mysql不支持这种方式。
怎么规避这个问题？
再加一层封装，如下：
mysql> delete from student where id not in (select minid from (select min(id) as minid from student group by name) b);
Query OK, 3 rows affected

mysql> select * from student;
6 g& D; t; I3 o3 H% }" x+ V+----+------+
5 a+ e4 H: m0 g( N# G  s| ID | NAME |
; J) ^. ^6 o" c: P; p+----+------+
| 11 | aa |
- B/ A" p! w6 y3 ~0 k8 z( h| 13 | bb |
( z5 m7 S( p/ l1 N% l' H* g6 Z; s, Z| 16 | cc |
+----+------+
3 rows in set

' v) i& e, E* o2 m; n& J3 q方法三例如：delete from bugdata where id not in (select minid from (select min(id) as minid from bugdata group by qxbh) b);
/ J: l. M0 j) g/ l6 y
; q* M/ I3 S( ?4 A5 D% e9 z! c