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PostgreSQL – CREATE PROCEDURE

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PostgreSQL CREATE PROCEDURE statement as the name suggests is used 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.

The following illustrates the basic syntax of the create procedure statement:

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

Let’s analyze the above 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. They 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;

Example:

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('Raju', 10000);

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

The following query will show the table data:

select * from accounts;

That depicts the result as shown below:

The following query 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;$$

Calling a stored procedure

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

call stored_procedure_name(argument_list);

Example:

The below statement invokes the transfer stored procedure to transfer $1, 000 from Raju’s account to Nikhil’s account:

call transfer(1, 2, 1000);

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

SELECT * FROM accounts;

Output:


Last Updated : 28 Aug, 2020
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