Postgresql IF statement

sql

conditional-logic

database-performance

error-handling

byAnton Shumikhin·Dec 30, 2024

IF logic in PostgreSQL is realized through a CASE expression in usual SQL queries or implemented as IF ... THEN ... ELSE blocks within a PL/pgSQL function.

For a brief SQL query using CASE:

SELECT (CASE WHEN quantity > 100 THEN 'Overstock' ELSE 'In stock' END) AS stock_status FROM product_table;

When applying it within a function, a PL/pgSQL IF block appears like:

CREATE FUNCTION stock_status() RETURNS text AS $$
BEGIN
  IF quantity > 100 THEN
    RETURN 'Overstock';
  ELSE
    RETURN 'In stock';
  END IF;
END;
$$ LANGUAGE plpgsql;

To introduce an IF statement on-the-fly without a pre-defined function, use DO:

DO $$
BEGIN
  IF (SELECT count(*) FROM customers) > 1000 THEN
    RAISE NOTICE 'Time to open another branch!';
  END IF;
END $$;

Checking for row existence with EXISTS before DELETE or INSERT:

DO $$
BEGIN
  IF NOT EXISTS(SELECT 1 FROM customers WHERE id = 123) THEN
    INSERT INTO customers(id, name) VALUES(123, 'John Doe');
  ELSE
    RAISE NOTICE 'Sorry, John. You seem to already exist.';
  END IF;
END $$;

To contend with concurrent data changes, use a WRITE-LOCK:

BEGIN;
LOCK TABLE customers IN SHARE ROW EXCLUSIVE MODE;
-- Now it's "queue or do not queue". There is no race.
COMMIT;

Break it down

Journey into PL/pgSQL

PL/pgSQL, the distinctive procedural language of PostgreSQL, enables complex conditional logic beyond the capabilities of standard inline SQL. It's your "Swiss Army Knife" in the toolbox of PostgreSQL functions, allowing for better readability and structured programming.

IF - The decision-maker in Transactions

The power of IF statements in PL/pgSQL lies in its decision-making capabilities during data manipulations. Think of it as your database's own "Sophia the Robot", intellectually processing before it acts:

Does a user exist? If not, let's create a new account.
Are there any products that have been inactive? If yes, let's delete them.
Is the inventory running low? If yes, let's update prices.

Syntax: The Devil's in the details

Accurate syntax and semicolon usage are mandatory in PL/pgSQL, as incorrect usage can be as painful as stepping on a LEGO. Including a semicolon at the end of each statement is a must. Moreover, careful positioning of BEGIN...END ensures your code doesn't go haywire.

IFs’ and BUTs' of Conditional Logic

A CASE expression can't replace the procedural logic of IF within a PostgreSQL function. Using DO blocks, you can execute anonymous code for conditional logic without requiring a full-fledged stored function - like having a "disposable" IF at your disposal.

Extra miles for Power Performance

EXISTS - Your Speedy Gonzalez

Use EXISTS to check if a row is present before doing heavyweight operations like DELETE or INSERT. This increases efficiency and enhances performance - like a warm-up before a marathon run!

Write-Locks - Your Concurrency Cop

Concurrency can be chaotic, like a busy intersection during rush hour! Sidestep race conditions and ensure data integrity by using LOCK TABLE. This confirms your IF logic gives green light to clean data.

Expert tricks up your sleeve

The Power of the RETURNING clause

Make use of the RETURNING clause when deleting or updating, to get back data from rows affected. Great for logging and for avoiding extra SELECT actions. It's like a receipt - proof of your transaction.

Conditional Triggers

With PostgreSQL, you can create triggers that call functions based on conditions. It's like setting up traps in a treasure hunt - certain actions invoke a response automatically as if by magic!

Error Handling - Your Safety Net

Don't forget to use EXCEPTION when you need to deal with potential errors during transactions. Like the goalkeeper in soccer, this feature catches fumbles that could cost you precious points!

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