MySQL takes the union of two query conditions and then queries

Problem analysis ：

such sql It sounds a little complicated , But when you think about it carefully, it's actually a very simple sql, First find out what you need sql
select dept_id from sys_dept sd where AREACODETYPE ='4' and AREACODE ='1111'

And then we sql The first condition is found , And put this sql Continue to query as a condition

select role_id from sys_role_dept srd,
(select dept_id from sys_dept sd where AREACODETYPE ='4' and AREACODE ='1111') a
where srd.dept_id = a.dept_id

Then you can continue the association query

select user_id from sys_user_role sur,(select role_id from sys_role_dept srd,
(select dept_id from sys_dept sd where AREACODETYPE ='4' and AREACODE ='1111') a
where srd.dept_id = a.dept_id) b where sur.role_id = b.role_id

Then the first result came out , Then we check the second result
Similar to the above logic , Finally, I won't explain the code block

select user_id from  sys_user_dept sud,
(select dept_id from sys_dept sd where AREACODETYPE ='4' and AREACODE ='1111') a
where sud.dept_id = a.dept_id

Problem solving

Then the results of the first column and the second column are different , But there are similarities , Then at this time, we can carry out union value .

select a.user_id as dept_id from (select user_id from sys_user_role sur,(select role_id from sys_role_dept srd,
(select dept_id from sys_dept sd where AREACODETYPE ='4' and AREACODE ='1111') a
where srd.dept_id = a.dept_id) b where sur.role_id = b.role_id) a,(select user_id from  sys_user_dept sud,
(select dept_id from sys_dept sd where AREACODETYPE ='4' and AREACODE ='1111') a
where sud.dept_id = a.dept_id)b where a.user_id = b.user_id

summary

The main thing is to use mysql Find out the data , Then treat the data as a new table file , And then we're working on it , Treat the data as another table file , But there are better ways to deal with it , I didn't write it here , You can provide some ideas ,
left join finger :
select * from a left join b on a.aid = b.bid
Take it out first a All data in the table , Then add and a,b Matching data
What is taken out at this time is :
1 a1 b1
2 a2 b2
3 a3 Null character
The result set of the left out join includes LEFT OUTER All rows of the left table specified in Clause , Not just the rows that the join columns match . If a row in the left table does not match a row in the right table , All selection list columns in the right table in the associated result set row are null .