How to declare and use a variable in postgresql?

@rollin 

In PostgreSQL, you can declare and use a variable within a PL/pgSQL block. Here's an example:

1. To declare a variable, use the DECLARE keyword followed by the variable name and data type:

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DO $$
DECLARE
    my_variable INTEGER;
BEGIN
    -- Your SQL statements here
END $$;

1. To assign a value to the variable, use the := operator:

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DO $$
DECLARE
    my_variable INTEGER;
BEGIN
    my_variable := 10;
END $$;

1. To use the variable in your SQL statements, simply reference it by name:

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DO $$
DECLARE
    my_variable INTEGER;
BEGIN
    my_variable := 10;

    SELECT * FROM some_table WHERE column = my_variable;
END $$;

Remember that variables declared in a PL/pgSQL block are only accessible within that block. To use a variable in a different block, you can pass it as a parameter or store it in a table.