Left join with condition

Question

Suppose I have these tables

create table bug (
    id int primary key, 
    name varchar(20)
)
create table blocking (
    pk int primary key,
    id int, 
    name varchar(20)
)

insert into bug values (1, 'bad name')
insert into bug values (2, 'bad condition')
insert into bug values (3, 'about box')
insert into blocking values (0, 1, 'qa bug')
insert into blocking values (1, 1, 'doc bug')
insert into blocking values (2, 2, 'doc bug')

and I'd like to join the tables on id columns and the result should be like this:

id          name                 blockingName
----------- -------------------- --------------------
1           bad name             qa bug
2           bad condition        NULL
3           about box            NULL

This means: I'd like to return all rows from #bug there should be only 'qa bug' value in column 'blockingName' or NULL (if no matching row in #blocking was found)

---

My naive select was like this:

select * from #bug t1 
    left join #blocking t2 on t1.id = t2.id
    where t2.name is null or t2.name = 'qa bug'

but this does not work, because it seems that the condition is first applied to #blocking table and then it is joined.

What is the simplest/typical solution for this problem? (I have a solution with nested select, but I hope there is something better)

Solution 1

Simply put the "qa bug" criteria in the join:

select t1.*, t2.name from #bug t1 
left join #blocking t2 on t1.id = t2.id AND t2.name = 'qa bug'

Solution 2

correct select is:

create table bug (
id int primary key, 
name varchar(20)
)
insert into bug values (1, 'bad name')
insert into bug values (2, 'bad condition')
insert into bug values (3, 'about box')

CREATE TABLE blocking
(
pk int IDENTITY(1,1)PRIMARY KEY ,
id int, 
name varchar(20)
)
insert into blocking values (1, 'qa bug')
insert into blocking values (1, 'doc bug')
insert into blocking values (2, 'doc bug')


select 
t1.id, t1.name,
(select  b.name from blocking b where b.id=t1.id and b.name='qa bug')
from bug t1 

Solution 3

select * 
from #bug t1 
left join #blocking t2 on t1.id = t2.id and t2.name = 'qa bug'

Solution 4

It looks like you want to select only one row from #blocking and join that to #bug. I would do:

select t1.id, t1.name, t2.name as `blockingName` 
from `#bug` t1
left join (select * from `#blocking` where name = "qa bug") t2
on t1.id = t2.id

Solution 5

make sure the inner query only returns one row. You may have to add a top 1 on it if it returns more than one.

select 
t1.id, t1.name,
(select  b.name from #blocking b where b.id=t1.id and b.name='qa bug')
from #bug t1 

View more solutions

Comments

[stej]

MySQL is preffered, but T-SQL is also OK.

[StevieG]

what are you trying to achieve? Is it all bugs that either don't have blockers or do have a 'qa bug' blocker?

[mathematical.coffee]

Firstly your row 2 won't be returned with NULL because it has a matching row in #blocking. Secondly, what SQL are you using? You have a mysql tag but this is invalid mysql syntax (the '#'), and asides from that your query works in mysql (see here).

[stej]

PK is not important for me here (but added some).

[mathematical.coffee]

Forget my first statement about row 2; I misunderstood you.

[stej]

@StevieG I need all bugs with their qa blockers if there is any (qa blocker). If there is no qa blocker, there should be NULL.

[stej]

@mathematical.coffee corrected table names

[StevieG]

@stej - so if a bug has no qa blocker, do you want to see a row for it in the results?

[stej]

@StevieG Yes, I want (that's the row with id 2 and 3)