Convert one row into multiple rows with fewer colu

问题:

I'd like to convert single rows into multiple rows in PostgreSQL, where some of the columns are removed. Here's an example of the current output:

name | st | ot | dt |
-----|----|----|----|
Fred | 8  | 2  | 3  |
Jane | 8  | 1  | 0  |
Samm | 8  | 0  | 6  |  
Alex | 8  | 0  | 0  |  

Using the following query:

SELECT
   name, st, ot, dt
FROM
   times;

And here's what I want:

name |  t | val |
-----|----|-----|
Fred | st |  8  |
Fred | ot |  2  |
Fred | dt |  3  |
Jane | st |  8  |
Jane | ot |  1  |
Samm | st |  8  |
Samm | dt |  6  |
Alex | st |  8  |

How can I modify the query to get the above desired output?

回答1:

SELECT
  times.name, x.t, x.val
FROM
  times cross join lateral (values('st',st),('ot',ot),('dt',dt)) as x(t,val)
WHERE
  x.val <> 0;

回答2:

The core problem is the reverse of a pivot / crosstab operation. Sometimes called "unpivot".

Basically, Abelisto's query is the way to go in Postgres 9.3 or later. Related:

• SELECT DISTINCT on multiple columns

You may want to use LEFT JOIN LATERAL ... ON u.val <> 0 to include names without valid values in the result (and shorten the syntax a bit).

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

If you have more than a few value columns (or varying lists of columns) you may want to use a function to build and execute the query automatically:

CREATE OR REPLACE FUNCTION f_unpivot_columns(VARIADIC _cols text[])
  RETURNS TABLE(name text, t text, val int) AS
$func$
BEGIN
   RETURN QUERY EXECUTE (
   SELECT
     'SELECT t.name, u.t, u.val
      FROM   times t
      LEFT   JOIN LATERAL (VALUES '
          || string_agg(format('(%L, t.%I)', c, c), ', ')
          || ') u(t, val) ON (u.val <> 0)'
   FROM   unnest(_cols) c
   );
END
$func$  LANGUAGE plpgsql;

Call:

SELECT * FROM f_unpivot_times_columns(VARIADIC '{st, ot, dt}');

Or:

SELECT * FROM f_unpivot_columns('ot', 'dt');

Columns names are provided as string literals and must be in correct (case-sensitive!) spelling with no extra double-quotes. See:

• Are PostgreSQL column names case-sensitive?

dbfiddle here

Related with more examples and explanation:

• How to unpivot a table in PostgreSQL

回答3:

One way:

with times(name , st , ot , dt) as(
select 'Fred',8  , 2  , 3  union all
select 'Jane',8  , 1  , 0  union all
select 'Samm',8  , 0  , 6  union all
select 'Alex',8  , 0  , 0  
)

select name, key as t, value::int  from 
(
    select name, json_build_object('st' ,st , 'ot',ot, 'dt',dt) as j
    from times
) t
join lateral json_each_text(j)
on true
where value <> '0'
-- order by name, case when key = 'st' then 0 when key = 'ot' then 1 when key = 'dt' then 2 end