How to fan out rows with a specified range of values?

Question

I have a table which essentially contains 3 columns: ID, FIRST, LAST. They're all integers.

I'm wondering if there's a way to use generate_series() to get a query result where for each row in the original table, there are many rows containing numbers between FIRST and LAST?

E.g. for a row with data (42, 5, 8) the output contains rows (42, 5), (42, 6), (42, 7) and (42, 8).

Answer 1

In Postgres 10 or later it can be as simple as:

SELECT id, generate_series(first, last) AS nr
FROM   tbl;

In older versions it is not advisable to have set-returning functions in the SELECT list. See:

• What is the expected behaviour for multiple set-returning functions in select clause?

Rows where generate_series() returns nothing are excluded from the result this way (e.g. when last < first or either is NULL). The same is true for the more explicit CROSS JOIN syntax variant demonstrated by a_horse. May or may not be desirable. If it's not, use LEFT JOIN .. ON true instead:

SELECT t.id, g.nr
FROM   tbl t
LEFT   JOIN LATERAL generate_series(t.first, t.last) g(nr) ON true;

(LATERAL is a noise word in this case, since it's assumed for functions in the FROM list.)

See:

• What is the difference between LATERAL and a subquery in PostgreSQL?

db<>fiddle here (demonstrating corner cases)

Answer 2

This can be achieved using generate_series()

with test_data (id, "first", "last") as (
   values (42, 5, 8), 
          (43, 6, 9)
)
select td.id, g.nr
from test_data td
  cross join lateral generate_series(td."first", td."last", 1) as g(nr)
order by td.id, g.nr;

Online example: https://rextester.com/TWE47868