How do I put an 'if clause' in an SQL string?

sql

conditional-statements

subqueries

join

byNikita Barsukov·Dec 19, 2024

You can implement if-then-else logic in SQL using the CASE statement:

SELECT CASE WHEN condition THEN true_value ELSE false_value END FROM table;

With condition acting as your if-condition, true_value is the output for when it's true and false_value when it's false. This approach works across all SQL environments.

Conditional updates in SQL

So, you want to UPDATE your table but under certain conditions? Enter stage right, the CASE statement:

UPDATE table_name
SET column_name = CASE 
                    WHEN condition THEN true_value 
                    ELSE column_name -- Original value remains if condition is false
                  END
WHERE certain_column = certain_value; -- as precise as a surgeon's scalpel

Don't forget that WHERE clause. It's your sniper targeting specific updates with precision.

Controlling updates via subqueries

Here's a cool trick: control updates with subqueries and save your precious system resources.

UPDATE target_table
SET status = 'Updated'
WHERE NOT EXISTS (
    SELECT 1 FROM related_table
    WHERE condition_to_check
    LIMIT 1  -- Like a bouncer at a club, letting just one row in
) AND target_condition;  -- your bouncer's friend checking IDs

Using NOT EXISTS, combined with LIMIT 1 for a performance boost, targets updates when a subquery returns no rows.

Enhancing efficiency with multi-table updates

Knock multiple tables with a single UPDATE through a well-planned JOIN.

UPDATE a
JOIN b ON a.b_id = b.id
SET a.status = 'New Status'
WHERE b.condition;  -- The bouncer of this VIP club

With this approach, you reduce the number of queries, saving calories for your database.

Futur-proofing queries with parameters

Get dynamic and sneak extra protection against SQL injection with parameters.

UPDATE table_name
SET column_name = CASE 
                    WHEN condition THEN ?  -- Yes, the `?` is not confused, it's a parameter
                    ELSE column_name
                  END
WHERE id = ?;  -- Another wildcard leaping into action

In your application, bind actual values to these '?' placeholders.

Visualizing conditional logic

Imagine coping with an unpredictable chef's mood when deciding today's special 🍲:

SELECT 
  CASE 
    WHEN [today's mood] THEN 'Egg Florentine'  -- When the chef's upbeat 🌞
    ELSE 'Baked Beans'   -- Otherwise, it's a beans day pair it with toast 🍞
  END as 'Today's Special'
...

And voila! An 'if clause' adapted to serving flexibility.

| 🌞 Chef’s mood good today? | Today's Special 🍲  |
| -------------------------- | --------- |
| Yes                        | Egg Florentine 🍳 |
| No                         | Baked Beans 🍞 |

Leveraging constructs for complex logic

Expand your SQL toolbox with control constructs like IF...ELSE in some databases like Transact-SQL.

IF (SELECT COUNT(*) FROM table WHERE condition) = 0
  BEGIN
    -- Insert ninja action when condition is met
    INSERT INTO table (columns) VALUES (values);  -- Your SQL action figure springing into action
  END;

Combining conditions and logical operators

Combine conditions using AND, OR, NOT for that bespoke query you need.

SELECT *
FROM table
WHERE (condition1 AND condition2) OR (condition3 AND NOT condition4);  -- Logic, meet SQL!

Addressing multiple conditions with `CASE`

It's a bird... It's a plane... No, It's the CASE statement addressing multiple conditions!

SELECT 
  CASE 
    WHEN condition1 THEN 'Result 1'
    WHEN condition2 THEN 'Result 2'
    ELSE 'Default Result'  -- Not all heroes wear capes
  END
FROM table;