Question

Two tables:

table1  
id | name
1  | aaa
2  | bbb
3  | ccc

table2
id | param
1  | x
1  | z
2  | x

table1.id = table2.id

I need to select everything from table1, with field param (1 if have, 0 if no, or null) by given param.

For example, by param “x”:

result:
id | name | param
1  | aaa  | 1
2  | bbb  | 1
3  | ccc  | 0

All I can do is just:

SELECT table1.id, table1.name, table2.param
FROM table1 
LEFT JOIN table2 ON table1.id = table2.id
WHERE table2.param = 'x'

But it doesn’t select all table1.
What is the right query for this task?

Answer 1

You are applying the filter AFTER the join. Try this instead:

SELECT table1.id, table1.name, table2.param
FROM table1 
LEFT JOIN table2 ON table1.id = table2.id and table2.param='x'