Question

I have an update query like following:

update table TABLE1 set COL1 = 'X' where COL2 = 'Y' ---1

Support the values ‘X’ and ‘Y’ are fetched from database now TABLE2. E.g.

select COL1, COL2 from TABLE2. ----2

I want to update table TABLE1 with values from TABLE2.

Just to make it more clear, assume that TABLE2 has following values:

Can you please help me in doing this in a single query!

I am using Oracle 11g.

Answer 1

For Oracle, this is the most basic way to do it:

update TABLE1
  set COL1 = (select TABLE2.COL1 from TABLE2 where TABLE2.COL2 = TABLE1.COL2)
  where COL2 IN (select TABLE2.COL2 from TABLE2);

This can be inefficient in some cases since it could execute a subquery for every row in TABLE1.

Depending on the declaration of primary key or unique constraints on both tables, you may be able to use the updateable inline-view method, which is probably more efficient:

update
  (select TABLE1.COL1 as T1C1, TABLE1.COL2 as T1C2, TABLE2.COL1 as T2C1
     from TABLE1 join TABLE2 on TABLE2.COL2 = TABLE1.COL2
  )
  set T1C1 = T2C1;