PostgreSQL combining tables

Question

I am a little confused about merging one table into another. My two tables looks like so:

Table A                                         Table B
id | name | likes | email | username            id | name | email   | username
1  | joe  |  3    |  null | null                1  | ben  | [email protected] | user


Result: Table A
id | name | likes | email   | username
1  | joe  | 3     | null    | null
2  | ben  | null  | [email protected] | user

My issue is that I do not want to overwrite the properties that are in the Table A. Is this a simple UNION?

John Woo · Accepted Answer · 2013-04-17 01:50:49Z

3

You can just execute INSERT INTO..SELECT statement,

INSERT INTO TableA(id, name, likes, email, username)
SELECT id, name, NULL AS likes, email, username
FROM TableB

What it does is it copies all records from TableB into TableA. But if you want the records for projection only, then a simple UNION will do just fine

SELECT id, name, likes, email, username FROM tableA
UNION
SELECT id, name, NULL AS likes, email, username FROM TableB

answered Apr 17, 2013 at 1:50

John Woo

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5 Comments

trev9065 Over a year ago

Okay that looks like what I was trying to write originally but what is going on with the SELECT id, name NULL AS ...?

John Woo Over a year ago

it sets NULL value for LIKES. You can alternatively do this, INSERT INTO TableA(id, name, email, username) SELECT id, name, email, username FROM TableB

trev9065 Over a year ago

Okay so whatever values I do not want to insert I can leave them out of the columns of the initial INSERT INTO statement?

John Woo Over a year ago

yes, if you leave them out, the values for that column will be set as NULL :D

trev9065 Over a year ago

Thanks for your help and quick response!

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