Query for crosstab view

问题:

I have a table in PostgreSQL like below:

--------------------------------------------------------------
Item1 | Item2 | Item3 | Item4 |Value1| Value2| Value3| Value4|
--------------------------------------------------------------

I want a query which will show this table like below:

ItemHead| ValueHead
---------------
Item1 | Value1|
---------------
Item2 | Value2| 
----------------
Item3 | Value3| 
----------------
Item4 | Value4|
---------------

回答1:

Use a single SELECT with a LATERAL join to a VALUES expression. That's shorter and faster than multiple SELECT statements:

SELECT v.*
FROM   tbl, LATERAL (
   VALUES
      (item1, value1)
    , (item2, value2)  -- data types must be compatible
    , (item3, value3)
    , (item4, value4)
   ) v ("ItemHead", "ValueHead");  -- your desired column names

Related:

• Convert one row into multiple rows with fewer columns
• SELECT DISTINCT on multiple columns
• Postgres: convert single row to multiple rows

Note: You added the tag crosstab. But the Postgres function crosstab() from the additional tablefunc module is used for pivoting, while this task is the opposite, sometimes referred to as unpivoting. Related:

• PostgreSQL Crosstab Query

回答2:

Simply use UNION ALL

SELECT item1, value1 FROM your_tab
UNION ALL
SELECT item2, value2 FROM your_tab
UNION ALL
SELECT item3, value3 FROM your_tab
UNION ALL
SELECT item4, value4 FROM your_tab