Question

In MS SQL Server, I create my scripts to use customizable variables:

DECLARE @somevariable int  
SELECT @somevariable = -1

INSERT INTO foo VALUES ( @somevariable )

I’ll then change the value of @somevariable at runtime, depending on the value that I want in the particular situation. Since it’s at the top of the script it’s easy to see and remember.

How do I do the same with the PostgreSQL client psql?

Answer 1

Postgres variables are created through the \set command, for example …

\set myvariable value

… and can then be substituted, for example, as …

SELECT * FROM :myvariable.table1;

… or …

SELECT * FROM table1 WHERE :myvariable IS NULL;

edit: As of psql 9.1, variables can be expanded in quotes as in:

\set myvariable value 

SELECT * FROM table1 WHERE column1 = :'myvariable';

In older versions of the psql client:

… If you want to use the variable as the value in a conditional string query, such as …

SELECT * FROM table1 WHERE column1 = ':myvariable';

… then you need to include the quotes in the variable itself as the above will not work. Instead define your variable as such …

\set myvariable 'value'

However, if, like me, you ran into a situation in which you wanted to make a string from an existing variable, I found the trick to be this …

\set quoted_myvariable '\'' :myvariable '\''

Now you have both a quoted and unquoted variable of the same string! And you can do something like this ….

INSERT INTO :myvariable.table1 SELECT * FROM table2 WHERE column1 = :quoted_myvariable;