SQL BEFORE

What is the SQL BEFORE trigger keyword?

BEFORE is a trigger-timing keyword that runs trigger code immediately before the associated DML statement executes.

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Compatible dialects for SQL BEFORE: Supported: PostgreSQL, MySQL, MariaDB, Oracle, SQLite (3.15+). Not supported: SQL Server.

SQL BEFORE Full Explanation

BEFORE appears in a CREATE TRIGGER statement to declare when the trigger fires. When specified, the database executes the trigger body before the triggering event (INSERT, UPDATE, DELETE) is applied to each affected row or once per statement, depending on FOR EACH clause. This timing lets you validate or transform data, enforce complex constraints, populate derived columns, or cancel the operation by raising an error. In row-level triggers, the pseudo-record NEW (and OLD for UPDATE/DELETE) is available and can be modified so the final DML statement uses the altered values. Statement-level BEFORE triggers can perform broader checks but cannot change individual rows. Not all databases support BEFORE; SQL Server, for example, offers INSTEAD OF triggers instead. The SQL standard (SQL:1999) introduced BEFORE/AFTER timing, but implementations vary in syntax details, such as delimiter requirements or ability to fire on TRUNCATE. Careful design is needed to avoid recursion, performance bottlenecks, and unintended side effects.

SQL BEFORE Syntax

CREATE TRIGGER trigger_name
BEFORE { INSERT | UPDATE [ OF column_list ] | DELETE }
ON table_name
[ FOR EACH { ROW | STATEMENT } ]
[ WHEN (condition) ]
BEGIN
    -- trigger body statements
END;

SQL BEFORE Parameters

Example Queries Using SQL BEFORE

-- 1. Auto-populate created_at before insert (PostgreSQL style)
CREATE TRIGGER set_created_at
BEFORE INSERT ON users
FOR EACH ROW
BEGIN
    NEW.created_at := NOW();
END;

-- 2. Validate price is positive before update (MySQL style)
DELIMITER //
CREATE TRIGGER check_product_price
BEFORE UPDATE ON products
FOR EACH ROW
BEGIN
    IF NEW.price <= 0 THEN
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Price must be positive';
    END IF;
END;//
DELIMITER ;

Expected Output Using SQL BEFORE

In example 1, every row inserted into users will have created_at set to the current timestamp automatically
In example 2, any UPDATE that tries to set price to zero or negative is aborted with an error

Use Cases with SQL BEFORE

Enforce business rules not expressible with CHECK constraints
Automatically set audit columns such as created_at or updated_at
Normalize data (e.g., trim whitespace, uppercase codes) before storage
Generate surrogate keys or sequence values in dialects that allow it
Block or log suspicious modifications before they reach the table

Common Mistakes with SQL BEFORE

Assuming SQL Server supports BEFORE triggers (it does not)
Forgetting to use FOR EACH ROW when you need row-level access
Trying to reference auto-generated values (like SERIAL) before they exist
Performing long-running logic that slows every DML statement
Creating mutually recursive triggers that cause infinite loops

First Introduced In

SQL:1999

Frequently Asked Questions

What does BEFORE do in a trigger?

BEFORE tells the database to run the trigger body prior to executing the triggering INSERT, UPDATE, or DELETE. This timing lets you validate or modify the data before it is stored.

How is BEFORE different from AFTER?

A BEFORE trigger runs first and can change NEW column values or cancel the statement. An AFTER trigger fires after the row is written, which is useful for logging but cannot alter the row that was just stored.

Is BEFORE supported in SQL Server?

No. SQL Server offers INSTEAD OF and AFTER triggers. Use INSTEAD OF to achieve similar pre-processing behavior.

Can one table have multiple BEFORE triggers?

Yes. Most databases allow multiple BEFORE triggers on the same table and event. Their execution order is either creation order (PostgreSQL, MySQL) or explicitly specified (Oracle).