mysql删除重复记录，保存Id最小的一条

admin

方法1：
; x! r2 N6 O; H3 I0 T; f8 o9 `1、创建一个临时表，选取需要的数据。
4 o) m6 a& c8 i7 y- M# F2、清空原表。
2 R8 V' i- g& c4 {$ _3、临时表数据导入到原表。
6 T% f' z# }$ z$ _" e4、删除临时表。
- s$ O& k2 I* H* j* b0 pmysql> select * from student;
+----+------+
" T$ v7 F2 ~+ Y- j2 P" n7 N* A| ID | NAME |
" B  w& d4 E: |6 Y* B+----+------+
| 11 | aa |
$ u- o, [+ p2 I3 u| 12 | aa |
| 13 | bb |
| 14 | bb |
' {+ H! s: l4 Q# I2 g: E| 15 | bb |
: w  @+ N5 D/ J$ F* M$ m| 16 | cc |
+----+------+
6 rows in set

mysql> create temporary table temp as select min(id),name from student group by name;
% ?: G9 {7 O4 s+ F4 `Query OK, 3 rows affected
Records: 3 Duplicates: 0 Warnings: 0

mysql> truncate table student;
Query OK, 0 rows affected

mysql> insert into student select * from temp;
Query OK, 3 rows affected
: R) Y( V$ y. ^: w1 z0 kRecords: 3 Duplicates: 0 Warnings: 0

mysql> select * from student;
" i' H! I- q( c* J2 `+----+------+
| ID | NAME |
& N5 D5 J5 ^+ @0 W+----+------+
$ W3 F' |9 C: v4 z+ e( D| 11 | aa |
| 13 | bb |
| 16 | cc |
+----+------+
3 rows in set

mysql> drop temporary table temp;
$ p- U" U* t9 i- a- u( f% I* y) OQuery OK, 0 rows affected
* R4 e# u; X/ u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;
Query OK, 3 rows affected
: @5 i2 |! A+ aRecords: 3 Duplicates: 0 Warnings: 0

mysql> delete from student where id not in (select minid from temp);
& k% e" s8 s' c6 f5 pQuery OK, 3 rows affected

mysql> select * from student;
+----+------+
5 X$ z* `. d9 V3 M& h1 Y| ID | NAME |
6 o: C  w" l% E+----+------+
| 11 | aa |
| 13 | bb |
| 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);
# @' d& F8 ~' _6 p' ~0 J$ t/ g执行报错：1093 - You can't specify target table 'student' for update in FROM clause
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：
, g% Q: ^" c! i( G. Zmysql> delete from student where id not in (select minid from (select min(id) as minid from student group by name) b);
. b3 L1 j+ m3 z4 k! ?% `8 \: CQuery OK, 3 rows affected

mysql> select * from student;
% o; R. D) _$ Y: b" \/ P( P  ~" m; o+----+------+
| ID | NAME |
+----+------+
| 11 | aa |
| 13 | bb |
| 16 | cc |
+----+------+
0 P) L' o" v! ^. D3 rows in set

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);
# t% o8 ~8 S: ?+ R7 U7 d
# R1 W" ~) j& p& v6 T4 ~- \
' K0 p* h1 o$ b