Question

SQL - Merging fields from two tables into one, skipping duplicates

Asked by: shacho

I have two tables, A and B.
A has two fields, KEY_A and VAL_A.
B has two fields, KEY_B and VAL_B.
Values in KEY_A and KEY_B may overlap.

I want to select all records from A and B into a new table, C with Fields KEY_C and VAL_C.  This means I need to combine the values of KEY_A and KEY_B into one field (KEY_C), weeding out the duplicates.  How can I do this?

Mike

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Asked On
2007-07-16 at 22:31:03ID22700605
Tags

sql

,

two

,

tables

,

one

,

merge

Topics

PostgreSQL Database

,

MySQL Server

,

Oracle Database

Participating Experts
3
Points
500
Comments
6

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Answers

 

by: sujith80Posted on 2007-07-16 at 23:35:15ID: 19502568

insert into C
select key_a, val_a from A
union
select key_b, val_b from B
/

 

by: ToFroPosted on 2007-07-16 at 23:48:15ID: 19502615

Use the INSERT INTO command combined with the UNION command to filter out the duplicates. MySQL Server doesn't support the SELECT ... INTO TABLE which you may have stumbled across on several SQL Tutorial sites:
http://dev.mysql.com/doc/refman/5.1/en/ansi-diff-select-into-table.html

INSERT INTO C(KEY_C, VAL_C)
SELECT KEY_A, VAL_A FROM A
UNION
SELECT KEY_B, VAL_B FROM B ORDER By KEY_A

Note that if the two duplicate KEY_? columns have different VALUE_? values. These rows will be treated as different and both included in the SELECT... UNION result.

 

by: prasanthi_kPosted on 2007-07-16 at 23:51:34ID: 19502631


insert into c
select key_a, val_a from A
union
select key_b, val_b from B
where key_b not in (select key_a from A)
/

 

by: ToFroPosted on 2007-07-17 at 00:06:10ID: 19502691

WHERE KEY_B NOT IN(SELECT KEY_A from A) solves the problem I mentioned where duplicate keys with different values are treated as different in the result. So forget that note.
Good one prasanthi_k.

 

by: sujith80Posted on 2007-07-17 at 00:48:50ID: 19502834

If the duplicates are defined on both the columns together then the UNION will remove the duplicates.
Else if, the duplicates are defined on the key column only then it should be known which value to be returned as the output val_a or val_b.

You can use the following query: change the second nvl clause to chose val_a or val_b.

insert into C
select nvl(key_a, key_b), nvl(val_a, val_b)
from A FULL OUTER JOIN B
on A.key_a = B.key_b
/

 

by: shachoPosted on 2007-07-30 at 18:58:41ID: 19597282

Thanks to all for your comments!

Mike

20120131-EE-VQP-002

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