Insert multiple rows in one table based on number in another table

Question

I am creating a database for the first time using Postgres 9.3 on MacOSX.

Let\'s say I have table A and B. A starts off as emp

Answer 1

Answer to original question

Postgres allows set-returning functions (SRF) to multiply rows. generate_series() is your friend:

INSERT INTO b (all_names, birthday)
SELECT names, current_date -- AS birthday ??
FROM  (SELECT names, generate_series(1, number) FROM a);

Since the introduction of LATERAL in Postgres 9.3 you can do stick to standard SQL: the SRF moves from the SELECT to the FROM list:

INSERT INTO b (all_names, birthday)
SELECT a.names, current_date -- AS birthday ??
FROM   a, generate_series(1, a.number) AS rn

LATERAL is implicit here, as explained in the manual:

LATERAL can also precede a function-call FROM item, but in this case it is a noise word, because the function expression can refer to earlier FROM items in any case.

Reverse operation

The above is the reverse operation (approximately) of a simple aggregate count():

INSERT INTO a (name, number)
SELECT all_names, count(*)
FROM   b
GROUP  BY 1;

... which fits your updated question.

Note a subtle difference between count(*) and count(all_names). The former counts all rows, no matter what, while the latter only counts rows where all_names IS NOT NULL. If your column all_names is defined as NOT NULL, both return the same, but count(*) is a bit shorter and faster.

About GROUP BY 1:

• GROUP BY + CASE statement