How Can I Use Script Variables Effectively in psql?

Using script variables in psql

psql (PostgreSQL client) allows you to create and use script variables using the set command. To define a variable, specify the name and value in the following format:

<code>\set variable_name value</code>

For example, to create a variable named "myvariable" with a value of "value":

<code>\set myvariable value</code>

You can then replace the variable in a query, table or condition, for example:

<code>SELECT * FROM :myvariable.table1;</code>

or

<code>SELECT * FROM table1 WHERE :myvariable IS NULL;</code>

Starting with psql 9.1, variables can also be expanded within quotes:

<code>\set myvariable value 

SELECT * FROM table1 WHERE column1 = :'myvariable';</code>

In earlier versions of psql, if you wanted to use a variable as a value in a conditional string query, you needed to include quotes in the variable definition:

<code>\set myvariable 'value'</code>

However, if you want to create a string from an existing variable, you can use the following trick:

<code>\set quoted_myvariable '\'' :myvariable '\''</code>

This will create two variables, one with quotes and one without quotes, that can be used together in queries like this:

<code>INSERT INTO :myvariable.table1 SELECT * FROM table2 WHERE column1 = :quoted_myvariable;</code>

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