Update one column based on three other column which have same value in another table

Question

I have two tables that have 3 fields with the same value and one relation field in the table1. Let me explain.

table1
------
id
column1
column2
column3
table2_id

table2
-------
id
column1
column2
column3

In table2; column1, column2 and column3 can have same value for different ids. For example:

table2
-------
1, 2, 3, 4
2, 2, 3, 4
3, 4, 5, 6

Ass you can see, only ids are different for the first two records. The others are the same. Also in table1:

table1
-------
1, 2, 3, 4, null
2, 4, 5, 6, null

I want to update table1's table2_id field(which are null in example) for only records in table1 that have one specific record for column1, column2, column3.

So I expect that:

table1
------
1, 2, 3, 4, null
2, 4, 5, 6, 3

first record will be still null because 2 different ids can be linked there. But for the second record, table2_id can only be '3'

How can I write the query for this update process?

Answer 1

You can solve it with an additional table or CTE to identify duplicate or genuine records. A GROUP BY or a join can be used to check the duplicates.

Please note that you also need to check for nulls in equations if the columns are nullable.

with GenuineData as (
   select
      min(t2.Id) id
   from
      Table2 t2
   group by
      t2.Column1,
      t2.Column2,
      t2.Column3
   having
      count(*) = 1
)
update t1 set
   t1.Table2Id = t2.Id
from
   Table1 t1
   join Table2 t2 on t2.Column1 = t1.Column1 and t2.Column2 = t1.Column2 and t2.Column3 = t1.Column3
   join GenuineData gd on gd.id = t2.Id

If the columns are nullable, you can use ISNULL() in the equations, or for better performance, an extended criteria for joining Table2 as below:

   join Table2 t2 on
      (t2.Column1 = t1.Column1 or (t1.Column1 is null and t2.Column1 is null))
      and (t2.Column2 = t1.Column2 or (t1.Column2 is null and t2.Column2 is null))
      and (t2.Column3 = t1.Column3 or (t1.Column3 is null and t2.Column3 is null))