Postgres 9.4 or newer
Obviously inspired by this post, Postgres 9.4 added the missing function(s):
Thanks to Laurence Rowe for the patch and Andrew Dunstan for committing!
To unnest the JSON array. Then use array_agg()
or an ARRAY constructor to build a Postgres array from it. Or string_agg()
to build a text
string.
Aggregate unnested elements per row in a LATERAL
or correlated subquery. Then original order is preserved and we don't need ORDER BY
, GROUP BY
or even a unique key in the outer query. See:
Replace 'json' with 'jsonb' for jsonb
in all following SQL code.
SELECT t.tbl_id, d.list
FROM tbl t
CROSS JOIN LATERAL (
SELECT string_agg(d.elem::text, ', ') AS list
FROM json_array_elements_text(t.data->'tags') AS d(elem)
) d;
Short syntax:
SELECT t.tbl_id, d.list
FROM tbl t, LATERAL (
SELECT string_agg(value::text, ', ') AS list
FROM json_array_elements_text(t.data->'tags') -- col name default: "value"
) d;
Related:
ARRAY constructor in correlated subquery:
SELECT tbl_id, ARRAY(SELECT json_array_elements_text(t.data->'tags')) AS txt_arr
FROM tbl t;
Related:
Subtle difference: null
elements are preserved in actual arrays. This is not possible in the above queries producing a text
string, which cannot contain null
values. The true representation is an array.
Function wrapper
For repeated use, to make this even simpler, encapsulate the logic in a function:
CREATE OR REPLACE FUNCTION json_arr2text_arr(_js json)
RETURNS text[] LANGUAGE sql IMMUTABLE PARALLEL SAFE AS
'SELECT ARRAY(SELECT json_array_elements_text(_js))';
Make it an SQL function, so it can be inlined in bigger queries.
Make it IMMUTABLE
(because it is) to avoid repeated evaluation in bigger queries and allow it in index expressions.
Make it PARALLEL SAFE
(in Postgres 9.6 or later!) to not stand in the way of parallelism. See:
Call:
SELECT tbl_id, json_arr2text_arr(data->'tags')
FROM tbl;
db<>fiddle here
Postgres 9.3 or older
Use the function json_array_elements()
. But we get double quoted strings from it.
Alternative query with aggregation in the outer query. CROSS JOIN
removes rows with missing or empty arrays. May also be useful for processing elements. We need a unique key to aggregate:
SELECT t.tbl_id, string_agg(d.elem::text, ', ') AS list
FROM tbl t
CROSS JOIN LATERAL json_array_elements(t.data->'tags') AS d(elem)
GROUP BY t.tbl_id;
ARRAY constructor, still with quoted strings:
SELECT tbl_id, ARRAY(SELECT json_array_elements(t.data->'tags')) AS quoted_txt_arr
FROM tbl t;
Note that null
is converted to the text value "null", unlike above. Incorrect, strictly speaking, and potentially ambiguous.
Poor man's unquoting with trim()
:
SELECT t.tbl_id, string_agg(trim(d.elem::text, '"'), ', ') AS list
FROM tbl t, json_array_elements(t.data->'tags') d(elem)
GROUP BY 1;
Retrieve a single row from tbl:
SELECT string_agg(trim(d.elem::text, '"'), ', ') AS list
FROM tbl t, json_array_elements(t.data->'tags') d(elem)
WHERE t.tbl_id = 1;
Strings form correlated subquery:
SELECT tbl_id, (SELECT string_agg(trim(value::text, '"'), ', ')
FROM json_array_elements(t.data->'tags')) AS list
FROM tbl t;
ARRAY constructor:
SELECT tbl_id, ARRAY(SELECT trim(value::text, '"')
FROM json_array_elements(t.data->'tags')) AS txt_arr
FROM tbl t;
Original (outdated) SQL Fiddle.
db<>fiddle here.
Related:
Notes (outdated since pg 9.4)
We would need a json_array_elements_text(json)
, the twin of json_array_elements(json)
to return proper text
values from a JSON array. But that seems to be missing from the provided arsenal of JSON functions. Or some other function to extract a text
value from a scalar JSON
value. I seem to be missing that one, too.
So I improvised with trim()
, but that will fail for non-trivial cases ...
I suggested that you use trigger arguments, but it's actually not necessary. You can use the automatic variables TG_TABLE_SCHEMA
and TG_TABLE_NAME
, or use TG_RELID
. These, alongside EXECUTE
for dynamic SQL, let you do what you want:
BEGIN
EXECUTE format('SELECT colname FROM %I', TG_RELID)
END;
or
BEGIN
EXECUTE format('SELECT colname FROM %I.%I', TG_TABLE_SCHEMA, TG_TABLE_NAME)
END;
(Of course these won't work as-is, since the SELECT
has no destination for the data. You have to use EXECUTE format(..) INTO ...
to store the result into a DECLARE
d variable), e.g.
DECLARE
_colvar integer;
BEGIN
EXECUTE format('SELECT colname FROM %I.%I', TG_TABLE_SCHEMA, TG_TABLE_NAME) INTO _colvar;
RAISE NOTICE 'colname value was %',_colvar;
END;
Best Answer
You need dynamic SQL for that, which carries the hazard of SQL injection.
However, done properly, this is safe against SQLi:
Table and column names are treated as case-sensitive. (You may want lower-case instead.) The type name is treated as case-insensitive, and any valid type name works.
Of course, this would raise an exception for the non-existent data type
String
you display. Try withtext
instead.Note the use of
format()
, the object identifier typeregclass
,quote_ident()
, the aggregation of columns in theLATERAL
subquery and theDO
command to execute dynamic SQL.Related: