题
MySQL中是否有oracle的rowID?
delete from my_table where rowid not in (select max(rowid) from my_table group by field1,field2)
我想使MySQL等同于此查询!!!
我要做的是:my_table没有主键..我正在尝试删除重复值并强加主键(field1,field2的复合键)..!
解决方案
在MySQL中,您通常使用会话变量来实现功能:
SELECT @rowid:=@rowid+1 as rowid
FROM table1, (SELECT @rowid:=0) as init
ORDER BY sorter_field
但是,您无法在桌子上删除桌子上的桌子。
UPD: :也就是说,您需要创建一个临时表,将范围子查询插入临时表,并通过与临时表连接(您需要一些唯一的行标识符):
CREATE TEMPORARY TABLE duplicates ...
INSERT INTO duplicates (rowid, field1, field2, some_row_uid)
SELECT
@rowid:=IF(@f1=field1 AND @f2=field2, @rowid+1, 0) as rowid,
@f1:=field1 as field1,
@f2:=field2 as field2,
some_row_uid
FROM testruns t, (SELECT @rowid:=NULL, @f1:=NULL, @f2:=NULL) as init
ORDER BY field1, field2 DESC;
DELETE FROM my_table USING my_table JOIN duplicates
ON my_table.some_row_uid = duplicates.some_row_uid AND duplicates.rowid > 0
由于这是一次时间操作,因此不应带来太多的开销。
其他提示
也许,我正在误读这个问题,但是您的查询(即使在Oracle中)并没有实现您的期望目标:
delete from my_table where rowid not in (select max(rowid) from
my_table group by field1,field2)
MySQL等效是
SELECT @rowid:=max(rowid) from my_table;
DELETE FROP my_table where rowid != @rowid;
除最后一条外,这将消除所有行。
要进行一次清理(删除重复记录),您可以执行此操作:
CREATE TABLE my_table2 SELECT distinct f1, f2, f3, etc from my_table;
DROP TABLE my_table;
ALTER TABLE my_table2 RENAME my_table;
然后添加Alter表所需的任何列和密钥。上面的代码可能需要丢弃您可能拥有的任何外国键。
您可以使用另一个派生表避免临时表:
DELETE FROM my_table USING my_table JOIN (
SELECT @rowid:=IF(@f1=field1 AND @f2=field2, @rowid+1, 0) as rowid,
@f1:=field1 as field1,
@f2:=field2 as field2,
some_row_uid
FROM testruns t, (SELECT @rowid:=NULL, @f1:=NULL, @f2:=NULL) as init
ORDER BY field1, field2 DESC) as duplicates
ON my_table.some_row_uid = duplicates.some_row_uid AND duplicates.rowid > 0
mysql> set @row_num = 0;
首先设置ROWID或行num使用如下:
mysql> SELECT @row_num := @row_num + 1 as row_number,id,name,salary FROM employee
ORDER BY salary;
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