Summary: in this tutorial, you will learn how to use the PostgreSQL CREATE PROCEDURE statement to create new stored procedures.

So far, you have learned how to define user-defined functions using the create function statement.

A drawback of user-defined functions is that they cannot execute transactions. In other words, inside a user-defined function, you cannot start a transaction, and commit or rollback it.

PostgreSQL 11 introduced stored procedures that support transactions.

To define a new stored procedure, you use the create procedure statement with the following syntax:

create [or replace] procedure procedure_name(parameter_list)
language plpgsql
as $$
declare
-- variable declaration
begin
-- stored procedure body
end; $$

In this syntax:

• First, specify the name of the stored procedure after the create procedure keywords.
• Second, define parameters for the stored procedure. A stored procedure can accept zero or more parameters.
• Third, specify plpgsql as the procedural language for the stored procedure. Note that you can use other procedural languages for the stored procedure such as SQL, C, etc.
• Finally, use the dollar-quoted string constant syntax to define the body of the stored procedure.

Parameters in stored procedures can have the in and inout modes but cannot have the out mode.

A stored procedure does not return a value. You cannot use the return statement with a value inside a store procedure like this:

return expression;

However, you can use the return statement without the expression to stop the stored procedure immediately:

return;

If you want to return a value from a stored procedure, you can use parameters with the inout mode.

We will use the following accounts table for the demonstration:

drop table if exists accounts;

create table accounts (
    id int generated by default as identity,
    name varchar(100) not null,
    balance dec(15,2) not null,
    primary key(id)
);

insert into accounts(name,balance)
values('Bob',10000);

insert into accounts(name,balance)
values('Alice',10000);

The following statement shows the data from the accounts table:

select * from accounts;

Output:

 id | name  | balance
----+-------+----------
  1 | Bob   | 10000.00
  2 | Alice | 10000.00
(2 rows)

The following example creates a stored procedure named transfer that transfers a specified amount of money from one account to another.

create or replace procedure transfer(
   sender int,
   receiver int,
   amount dec
)
language plpgsql
as $$
begin
    -- subtracting the amount from the sender's account
    update accounts
    set balance = balance - amount
    where id = sender;

    -- adding the amount to the receiver's account
    update accounts
    set balance = balance + amount
    where id = receiver;

    commit;
end;$$;

To call a stored procedure, you use the CALL statement as follows:

call stored_procedure_name(argument_list);

For example, this statement invokes the transfer stored procedure to transfer $1,000 from Bob’s account to Alice’s account.

call transfer(1,2,1000);

The following statement verifies the data in the accounts table after the transfer:

SELECT * FROM accounts;

Output:

 id | name  | balance
----+-------+----------
  1 | Bob   |  9000.00
  2 | Alice | 11000.00
(2 rows)

The output shows that the transfer has been successful.

• Use create procedure statement to define a new stored procedure.
• Use the call statement to invoke a stored procedure.