一对多加入，但只有第一行

Question

I have 2 tables that I need to join via ID without getting the duplicate values For ID, InfoA, and InfoB. I do not need the data in column InfoB2. When I join the table on ID because it is a 1 to many join I end up with duplicate values and want to get rid of those. I only want ID, InfoA, and InfoB without the duplicates. Any ideas?

Example: TableA:

| ID      |   InfoA  |
|   1     |   animals|
|   2     |   plants |

TableB:

|     ID  |   InfoB  | InfoB2   |
|   1     |   A      |   X      |
|   1     |   A      |   Y      |
|   1     |   A      |   Z      |
|   2     |   B      |   X      |
|   2     |   B      |   Y      |
|   2     |   B      |   Z      |

Doing a normal join, because it is 1 to many I get this but do not want the duplicates. I don't want this:

|     ID  |   InfoB  |  InfoB   |
|   1     |   animals|   A      |
|   1     |   animals|   A      |
|   1     |   animals|   A      |
|   2     |   plants |   B      |
|   2     |   plants |   B      |
|   2     |   plants |   B      |

My goal is to get this (note I do not need column InfoB2):

|     ID  |   InfoA  |   InfoB  |
|   1     |   animals|   A      |
|   2     |   plants |   B      |

Answer 1

You could use the distinct keyword:

SELECT DISTINCT a.id, infoa, infob
FROM   tablea a
JOIN   tableb b ON a.id = b.id

Answer 2

The fastest way is likely to be:

select a.*,
       (select b.infob from tableb b on a.id = b.id limit 1)
from tablea a;

For performance, you would want an index on tableb(id, infob) .