PostgreSQL unnest with empty array

Question

I use postgreSQL 9.1. In my database there is a table which looks like

id | ... | values
-----------------------
1  | ... | {1,2,3}
2  | ... | {}

where id is an integer and values is an integer array. The arrays can be empty.

I need to unnest this list. If I query

select id, ..., unnest(values)
from table

I get three rows for id = 1 (as expected) and no lines for id = 2. Is there a way to get a result like

id  | ... | unnest
-------------------
1   | ... | 1
1   | ... | 2
1   | ... | 3
2   | ... | null

i.e. a query which also contains the lines which have an empty array?

score 27 · Accepted Answer · 2018-09-11 17:11:54Z

27

select id, 
       case 
         when int_values is null or array_length(int_values,1) is null then null
         else unnest(int_values)
       end as value
from the_table;

(note that I renamed the column values to int_values as values is a reserved word and should not be used as a column name).

SQLFiddle: http://sqlfiddle.com/#!1/a0bb4/1

Postgres 10 does not allow to use unnest() like that any more.

You need to use a lateral join:

select id, t.i
from the_table
   cross join lateral unnest(coalesce(nullif(int_values,'{}'),array[null::int])) as t(i);

Online example: http://rextester.com/ALNX23313

It can be simplified even further when using a left join instead of the cross join:

select id, t.i
from the_table
 left join lateral unnest(int_values) as t(i) on true;

Online example: http://rextester.com/VBO52351

edited Sep 11, 2018 at 17:11

answered Mar 2, 2013 at 14:24

user330315

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Comments

Eduardo · Accepted Answer · 2018-04-09 15:37:00Z

20

This works on Postgres 10 also:

SELECT id, UNNEST(CASE WHEN "values" <> '{}' THEN "values" ELSE '{null}' END)

answered Apr 9, 2018 at 15:37

Eduardo

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1 Comment

David Farrell Over a year ago

I ended up going with the lateral join, but I really appreciate this solution as sometimes I just want to inline my unnests and not join them ... Additionally a cursory explain suggested that this approach may be tad more efficient than the lateral join ... Thanks for sharing !

Erwin Brandstetter · Accepted Answer · 2024-09-01 18:40:45Z

6

Since Postgres 10

A LATERAL subquery is the clean, versatile solution, and works since Postgres 9.3, where LATERAL was added:

SELECT t.id, v.value
FROM   tbl t
LEFT   JOIN LATERAL unnest(t.values) AS v(value) ON true;

Or, with minimal syntax:

SELECT id, value
FROM   tbl
LEFT   JOIN LATERAL unnest(values) value ON true;

fiddle

See:

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

Original answer

This works up to Postgres 9.6. The behavior of set-returning functions in the SELECT list was sanitized in Postgres 10, the short syntax below stops working. See:

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

Flip the logic in the currently accepted answer by @a_horse:

SELECT id, CASE WHEN values <> '{}' THEN unnest(values) END AS value
FROM   tbl;

fiddle

This returns a row with NULL in value for an empty array as well as for a NULL array, because only an array with elements in it produces true in the test values <> '{}'.

Works for arrays of any type, since the literal '{}' is automatically coerced to a matching type.

Without explicit ELSE branch, CASE returns NULL, which is what we want anyway.

Arrays with a NULL elements will return rows regardless.

However, I found an anomaly which I addressed in a related question:

NULL emements lost when casting result of unnest()

Turned out to be a bug. It was fixed after my report for Postgres 9.3+.

edited Sep 1, 2024 at 18:40

answered May 8, 2013 at 22:43

Erwin Brandstetter

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2 Comments

W1M0R Over a year ago

Trying this solution in postgres 12, I get: set-returning functions are not allowed in CASE LINE 5: case when values <> '{}' then unnest(values... ^ HINT: You might be able to move the set-returning function into a LATERAL FROM item

Erwin Brandstetter Over a year ago

@W1M0R: The behavior of SRFs was sanitized in Postgres 10. As a side-effect, my old short syntax stops working. I updated to clarify.

Clodoaldo Neto · Accepted Answer · 2013-03-02 15:31:35Z

1

select id,
    unnest (
        "values"
        ||
        (array[null]::integer[])[1:(array_upper("values", 1) is null)::integer]
    )
from "table"

answered Mar 2, 2013 at 15:31

Clodoaldo Neto

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Comments

vyegorov · Accepted Answer · 2013-03-02 14:18:04Z

0

You will need to use self LEFT JOIN, like this (also on SQL Fiddle):

SELECT t.id, u.u
  FROM tab t
  LEFT JOIN (SELECT id, unnest(vals) u FROM tab) u
    USING (id);

Note, that for bigger tables query will be performing badly.

answered Mar 2, 2013 at 14:18

vyegorov

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