Want to copy the data form one table to another.

Question

I have two tables named COMP_TRAINING_T and TRNG_CMTS_COMMENTS.
Now i feel the TRNG_CMTS_COMMENTS is needed so i am moving a column from this table to COMP_TRAINING_T.
I already have some data in both tables. I have add the column in COMP_TRAINING_T and want to update this table as follows
update COMP_TRAINING_T a
SET COMP_TRNG_COMMENTS=
( Select COMP_TRNG_CMTS_COMMENTS from comp_trng_cmts_t b
where a.comp_trng_finance_nbr= b.comp_trng_cmts_finance_nbr and
a.comp_trng_audit_userid=b.comp_trng_cmts_audit_userid and
a.comp_trng_date= b.comp_trng_cmts_date and
a.comp_trng_userid= b.comp_trng_cmts_userid)
but it is updating the rows correctly for the data it found from the COMP_TRNG_CMTS_COMMENTS table but for other rows it is setting it as Null value.
How to rewrite this sql.
Thanks

Answer 1

Hi,
Welcome to the forum!
You're doing an UPDATE statement without a WHERE clause, so every row in the destination table will be updated. If the correlated sib-query doesn't find any matching rows in the source table, there's no error, but NULL is returned, so some rows in the destination table may get set to NULL.
To avoid that, you can use MERGE:
MERGE INTO     comp_training_t     a
USING (
      SELECT     comp_trng_cmts_comments
      ,          comp_trng_cmts_finance_nbr
      ,          comp_trng_cmts_audit_userid
      ,          comp_trng_cmts_date
      ,          comp_trng_cmts_userid
      FROM     comp_trng_cmts_t
      )  b
ON    (          a.comp_trng_finance_nbr      = b.comp_trng_cmts_finance_nbr
      AND     a.comp_trng_audit_userid = b.comp_trng_cmts_audit_userid
      AND     a.comp_trng_date      = b.comp_trng_cmts_date
      AND     a.comp_trng_userid      = b.comp_trng_cmts_userid
WHEN MATCHED THEN UPDATE
SET   a.comp_trng_comments     = b.comp_trng_cmts_comments
;MERGE tests (using the ON conditions) whether or not ther is a match between the destination table and the source sub-query.
If there is a match, it can do an UPDATE.
If there is no match, it can do an INSERT. In this case, we don't want to insert any new rows, so we just omit the WHEN NOT MATCHED clause.
Notice that the USING sub-query returns all the columns from the source table that play any role in this job.
The ON clause contains all the correlation conditions that were in your original statement.
The alternatives, using UPDATE are:
(a) Update all rows, but use NVL to set values to themselves if the sub-query returns NULL
(b) Add a WHERE clause that repeats the correlated sub-query.
Neither is efficient, and (b) is a maintenance problem, too: if tyou ever change the conditions, you have to make the same change in two places.
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