即使有3行数据，Postgresql也会选择5行

Question

如果有3行数据，如何选择5行，接下来的2行应为空行。

Answer 1

您可以将结果与空行结合使用，该行应具有与表相同类型的列。例如：

with my_table(id, str, a_date) as (
values
    (1, 'first', '2017-09-01'::date),
    (2, 'second', '2017-09-02'),
    (3, 'third', '2017-09-03')
)

select *
from my_table

union all

select null::int, null::text, null::date
from generate_series(1, 5) -- generate 5 empty rows

order by id nulls last
limit 5

 id |  str   |   a_date   
----+--------+------------
  1 | first  | 2017-09-01
  2 | second | 2017-09-02
  3 | third  | 2017-09-03
    |        | 
    |        | 
(5 rows)    

Answer 2

您需要一个包含5个数字的表，然后为表分配行号并执行左连接。

cte是数据，最后是查询需要它：

<强> SQL DEMO

WITH all5 as (
    SELECT *
    FROM (VALUES(1), (2), (3), (4), (5)) AS t (ID)
), some2 as (
    SELECT *
    FROM (VALUES('first'), ('second')) AS t (value)
)
SELECT * 
FROM all5
LEFT JOIN (SELECT row_number() OVER (ORDER BY value) as rn, *
           FROM some2) as t
  ON all5.ID = t.rn          

<强>输出