mysql8.0改5.7细节

先展示演示用表：

一、不接受子查询的 from 后所接的子查询里有主查询字段

这是可以执行的，因为主查询字段a.id没跟在from后

select (select b.val from tab_b b where b.id=a.id) from tab_a a
结果：
a
bb
c

而这种则不行，因为a.id所在的查询语句为from后的子查询

select (select b.val from (select * from tab_b b0 where b0.id=a.id) b) from tab_a a
结果：
> 1054 - Unknown column 'a.id' in 'where clause'

所以在做 多个id拼接的字符串转换 (如：多id字符串转中文) 的查询时，可以考虑将主查询的约束条件字段放到子查询的where中：

 1 select
 2 (SELECT GROUP_CONCAT(b.id) from tab_b b
 3   INNER JOIN
 4   (SELECT
 5   help_topic_id+1 as ID,
 6   SUBSTRING_INDEX(SUBSTRING_INDEX(a.val,';',help_topic_id+1),';',-1) AS `value`
 7   FROM
 8   mysql.help_topic
 9   WHERE
10   help_topic_id < LENGTH(a.val)-LENGTH(REPLACE(a.val,';',''))+1) t
11 on b.val = t.`value` and t.`value` != '')
12 from tab_a a where a.id=2
13 
14 修改为：
15 
16 select
17 (SELECT GROUP_CONCAT(b.id) from tab_b b where FIND_IN_SET(b.val, REPLACE(a.val,';',','))>0 and IFNULL(b.val,'')!='')
18 from tab_a a where a.id=2

注意：
1,当sql语句中用此语句做值转换时，如果有要求顺序，则不可以使用 FIND_IN_SET 方式改写，可以使用代码的方式实现；

2, FIND_IN_SET 的分隔符是 ',' 若字符串使用的分隔符是 ';' 请使用 REPLACE(xxx,';',',') 进行转换；

二、没有row_number()等窗口函数

select row_number() over (ORDER BY a.id desc) as row_number, a.id, a.val from tab_a a
结果：
> 1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '(ORDER BY a.id), a.id, a.val from tab_a a' at line 1

修改为：

select @row_number := @row_number+1 as row_number, a.id, a.val from tab_a a, (select @row_number := 0) tt order by a.id desc

注意：是否是分组后再进行计算

如： SELECT row_number() over (PARTITION BY yyy ORDER BY zzz) as max_xxx FROM t

修改为：

select t.*, tab.seq
FROM t as t
INNER JOIN (
　　select a.id, a.zzz, count(1) as seq
　　from t as a
　　left join t as b
　　　　on a.yyy=b.yyy
　　　　and b.sss= 'sss'
　　　　and a.id>=b.id
　　where a.sss= 'sss'
　　group by a.id,a.yyy
　　ORDER BY a.zzz
) tab on tab.id = t.info_uid
WHERE t.sss= 'sss';
（参考分组排序）

上面是排序的2种情况：

1，直接排序，  将结果整体直接排序

2，分组排序，  将结果先按某字段分组，再组内排序

类似的还有 计数、分组计数，最大值、分组最大值 等，改用临时表实现（思路是先分组，再用临时表关联处理）：

修改模板：

drop temporary table if exists `temp_list`;
create temporary table `temp_list`(
　　
);
drop temporary table if exists `temp_group_count`;
create temporary table `temp_group_count`(
   select file_type_id, count(1) as count from temp_list group by file_type_id
);
select f.*, c.count from temp_list f inner join temp_group_count c on f.file_type_id = c.file_type_id order by file_type_id desc;
drop temporary table if exists `temp_list`;
drop temporary table if exists `temp_group_count`;

三、不支持select CAST(1 as float)

select CAST(1 as float)
> 1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'float)' at line 1

修改为：

select CAST(1 as DECIMAL)

四、不接受 insert into xxx(…) (select … union select … union select …)

INSERT into tab_a(id,val) 
(
select 4,'a'
UNION
select 5,'b'
)
结果为：
> 1064 -You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'UNION
select 5,'b'
)' at line 4

 修改为： INSERT into tab_a(id,val) values(4,'a'),(5,'b')

五、删除语句主表不支持别名，但可以在子查询中引用 表名.字段 做关联

DELETE FROM tab_a a where a.id=1
结果为：
> 1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'a where a.id=1' at line 1

修改为：

DELETE FROM tab_a  where id=1

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