Question

I have 3 tables:

‘CouponType’ table:

 AutoID         Code        Name

    1         CouT001     SunCoupon     
    2         CouT002     GdFriCoupon     
    3         CouT003     1for1Coupon  

‘CouponIssued’ table:

 AutoID              CouponNo        CouponType_AutoID  

    1                 Co001               1 
    2                 Co002               1
    3                 Co003               1                   
    4                 Co004               2
    5                 Co005               2 
    6                 Co006               2    

‘CouponUsed’ table:

  AutoID     Coupon_AutoID

    1              2  
    2              3  
    3              5   

I am trying to join 3 tables together using this query below but apparently I am not getting right values for CouponIssued column:

select CouponType.AutoID, Code, Name, Count(CouponIssued.CouponType_AutoID), count(CouponUsed.Coupon_AutoID)
from (CouponType left join CouponIssued
on (CouponType.AutoID = CouponIssued.CouponType_AutoID))
left join CouponUsed
on  (couponUsed.Coupon_AutoID = CouponIssued.AutoID)
group by CouponType.AutoID, code, name
order by code

The expected result should be like:

**Auto ID         Code           Name             Issued            used**

1                 CouT001       SunCoupon           3                  2

2                 CouT002       GdFriCoupon         3                  1

3                 CouT003       1for1Coupon         0                  0

Thanks!

Answer 1

SELECT t.AutoID
      ,t.Code
      ,t.Name
      ,count(i.CouponType_AutoID) AS issued
      ,count(u.Coupon_AutoID) AS used
FROM   CouponType t
LEFT   JOIN CouponIssued i ON i.CouponType_AutoID = t.AutoID
LEFT   JOIN CouponUsed u ON u.Coupon_AutoID = i.AutoID
GROUP  BY 1,2,3;

You might consider using less confusing names for your table columns. I have made very good experiences with using the same name for the same data across tables (as far as sensible).

In your example, AutoID is used for three different columns, two of which appear a second time in another table under a different name. This would still make sense if Coupon_AutoID was named CouponIssued_AutoID instead.