Question

I have 3 tables like this:

table_1

+-ID-+-table_3_id-+
|  1 |          1 |
|  2 |          1 |
|  3 |          2 |
+----+------------+

table_2

+-ID-+-table_1_id-+-name--+-value-+
|  1 |          1 | Name1 | Data1 |
|  2 |          1 | Name2 | Data2 |
|  3 |          1 | Name3 | Data3 |
|  4 |          2 | Name1 | Data1 |
|  5 |          2 | Name2 | Data4 |
|  6 |          2 | Name3 | Data5 |
|  7 |          3 | Name1 | Data6 |
|  8 |          3 | Name2 | Data2 |
+----+------------+-------+-------+

table 3 consists of IDs and other data that is irrelevant to this question. However, I need to be able to filter on table_3_id.

This is what I need:
Table 2 has multiple rows that have information of the rows in table_1. I need to have a query that checks if there are duplicates in the ‘data’ column, which have the same ‘name’. The result I need would be this (with a WHERE table_3_id = 1):

+-table_3_id-+-name--+-value-+-duplicate-+
|          1 | Name1 | Data1 | true      |
|          1 | Name2 | Data2 | false     |
|          1 | Name3 | Data3 | false     |
+------------+-------+-------+-----------+

Or, if possible, only return the data from table_2 where it actually is a duplicate. The ‘duplicate’ field can be a count too, since I do know the amount of rows that have the same table_3_id.

I hope I have made my question clear enough. If it is not clear enough I will try to improve it. I have tried it with joins and subqueries, but my knowledge of SQL isn’t enough to make an advanced query like this. I’d prefer this in a single query instead of multiple in PHP.

Answer 1

You can use something along the lines of the below:

select Name
from table
group by name
having count(*) > 1