In Postgresql how to order by date while keeping custom date format

问题

The issue is that using to_char will turn order by date into order by ascii. Example:

SELECT foo, bar FROM baz ORDER BY foo;

I would like to format foo using to_char, but doing this will affect the order:

SELECT to_char(foo,'dd/MM/yyyy') as foo, bar FROM baz ORDER BY foo;

Because foo now is of type text. There is a way to correctly do this? Or only in the code?

回答1:

You can use a different alias for the formatted column:

SELECT to_char(foo,'dd/MM/yyyy') as formatted_foo, 
       bar 
FROM baz 
ORDER BY foo;

As an alternative if you need to keep the foo alias:

select foo,
       bar
from (
  SELECT to_char(foo,'dd/MM/yyyy') as foo,
         foo as foo_date 
         bar 
  FROM baz 
) t
order by foo_date

回答2:

The proper and simple solution is:

SELECT to_char(b.foo,'dd/MM/yyyy') as foo, b.bar
FROM   baz b
ORDER  BY b.foo;

The formatted date column foo is a completely new column for the query planner, that happens to conflict with the table column foo. In ORDER BY and GROUP BY clauses, output columns' names take precedence over table columns. The unqualified name foo would refer to the output column.

To use the original table column in the ORDER BY clause, just table-qualify the column.

I table-qualified all table columns to clarify my point. Would only be required in the ORDER BY clause in this case. Table alias b is just for convenience.

回答3:

What is wrong with

SELECT foo AS foo_date, to_char(foo,'dd/MM/yyyy') AS foo, bar 
FROM baz 
ORDER BY foo_date;

回答4:

An alternative to not pulling foo twice would be:

SELECT
  to_char(foo,'dd/MM/yyyy') AS foo -- Alias precedes original column name
  ,bar
FROM
  baz
ORDER BY
  CAST(foo AS DATE) -- Explicit cast ensures date order instead of text
;

The native PostgreSQL version of that would be:

SELECT
  to_char(foo,'dd/MM/yyyy') AS foo -- Alias precedes original column name
  ,bar
FROM
  baz
ORDER BY
  foo::DATE -- Explicit cast ensures date order instead of text
;

来源：https://stackoverflow.com/questions/11232804/in-postgresql-how-to-order-by-date-while-keeping-custom-date-format

标签

postgresql

date

formatting

sql-order-by