Mysql左加入力分组依据

Question

I have notice that when I use a left join to another table, My data duplicates itself and I don't have more option than make a group by so that I could have the correct rows.

SELECT * from tbla a left join tblb b on a.id=b.id_a
group by a.id

Are there a way to make a left join returning correct rows with no duplicate?

Answer 1

depend on your data, you are probably having duplicated id on tablea so you are creating duplicates.

without know your data you have two options

SELECT *
FROM 
     (
      SELECT DISTINCT <tablA_fields> 
      from tbla a 
     ) as A
left join      (
      SELECT DISTINCT <tablB_fields> 
      from tblb b
     ) as B
tblb b on A.id = B.id_a

Second option use group by instead on each sub select.