Question

I am trying to get data from two related tables in a single query.
Tables look like this:

Table 1:

--------------------------
| ID        | username   |
--------------------------
| 1         | user1      |
| 2         | user2      |
| 3         | user3      |
| 4         | user4      |
--------------------------

Table 2:

----------------------------------------------------
| ID        | user_id    | key        | value      |
----------------------------------------------------
| 1         | 1          | key1       | value1     |
| 2         | 1          | key2       | value2     |
| 3         | 2          | key2       | value3     |
| 4         | 3          | key3       | value4     |
| 5         | 4          | key1       | value5     |
| 6         | 4          | key3       | value5     |
----------------------------------------------------

I want to get a result including all rows from Table 1 and key-value pairs from table 2 where key = key1. If the key is not present in Table 1 for a given row from table 1, fill it with NULL. Expected result example:

----------------------------------------------------
| user_id   | username   | key        | value      |
----------------------------------------------------
| 1         | user1      | key1       | value1     |
| 2         | user2      | NULL       | NULL       |
| 3         | user3      | NULL       | NULL       |
| 4         | user4      | key1       | value5     |
----------------------------------------------------

Alternatively – all the rows from Table 1 which don’t contain key-value pair for key=key1 in Table 2.

I tried standard SELECT from 2 tables and SELECT from Table 1 with JOIN from Table 2, but all I get is all rows from Table 1 which contain key-value pair with key=key1 in Table 2 and this is exactly the opposite of what I want.

Any suggestions?

Answer 1

select
      t1.id,
      t1.username,
      t2.key,
      t2.value
   from
      table1 t1
         left join table2 t2
            on t1.id = t2.user_id
           and t2.key = 'key1'

For your “Alternative” to get all that DO NOT have, just add a where clause..

where t2.user_id IS NULL