Update rows in one table with data from another table based on one column in each being equal

Question

Update many rows into one table from another table based on one column in each being equal (user_id).

both tables have a user_id column. Need to insert data from t2 into t1 when the user_id column are equal.

Answer 1

update 
  table1 t1
set
  (
    t1.column1, 
    t1.column2
      ) = (
    select
      t2.column1, 
      t2.column2
    from
      table2  t2
    where
      t2.column1 = t1.column1
     )
    where exists (
      select 
        null
      from 
        table2 t2
      where 
        t2.column1 = t1.column1
      );

Or this (if t2.column1 <=> t1.column1 are many to one and anyone of them is good):

update 
  table1 t1
set
  (
    t1.column1, 
    t1.column2
      ) = (
    select
      t2.column1, 
      t2.column2
    from
      table2  t2
    where
      t2.column1 = t1.column1
    and
      rownum = 1    
     )
    where exists (
      select 
        null
      from 
        table2 t2
      where 
        t2.column1 = t1.column1
      ); 

Answer 2

If you want to update matching rows in t1 with data from t2 then:

update t1
set (c1, c2, c3) = 
(select c1, c2, c3 from t2
 where t2.user_id = t1.user_id)
where exists
(select * from t2
 where t2.user_id = t1.user_id)

The "where exists" part it to prevent updating the t1 columns to null where no match exists.

Answer 3

merge into t2 t2 
using (select * from t1) t1
on (t2.user_id = t1.user_id)
when matched then update
set
   t2.c1 = t1.c1
,  t2.c2 = t1.c2

Answer 4

It's not an insert if the record already exists in t1 (the user_id matches) unless you are happy to create duplicate user_id's.

You might want an update?

UPDATE t1
   SET <t1.col_list> = (SELECT <t2.col_list>
                          FROM t2
                         WHERE t2.user_id = t1.user_id)
 WHERE EXISTS
      (SELECT 1
         FROM t2
        WHERE t1.user_id = t2.user_id);

Hope it helps...

Answer 5

You Could always use and leave out the "when not matched section"

merge into table1 FromTable   
   using table2 ToTable
     on     ( FromTable.field1 = ToTable.field1
          and  FromTable.field2 =ToTable.field2)
when Matched then
update set 
  ToTable.fieldr = FromTable.fieldx,
  ToTable.fields = FromTable.fieldy, 
  ToTable.fieldt =  FromTable.fieldz)
when not matched then
  insert  (ToTable.field1,
       ToTable.field2,
       ToTable.fieldr,
       ToTable.fields,
       ToTable.fieldt)
  values (FromTable.field1,
         FromTable.field2,
         FromTable.fieldx,
         FromTable.fieldy,
         FromTable.fieldz);