SQL trigger, check if certain value was entered

Question

INSERT INTO table_name (column1, column2, column3,...) VALUES (value1, value2, value3,...) 

Now what to check:

If inserted value1 = null, change it to 0

How to do it via trigger? I googled for examples and I have never ever done a trigger, so it is rather confusing.

So far got only this:

CREATE TRIGGER testTrigger ON myTable AFTER INSERT, UPDATE, DELETE 

Answer 1

You can add default value . This is how it's done for a new column. For existing one you should add constraint. Check Update 2

ALTER TABLE table_name  ADD column1 int NOT NULL DEFAULT(0) 

Add a column with a default value to an existing table in SQL Server

UPDATE:

To set default value, you should update NULL values at first.

UPDATE table_name  SET column1 = 0 WHERE column1 IS NULL 

UPDATE 2:

Try adding constraint

ALTER TABLE table_name  ADD CONSTRAINT DF_column1 DEFAULT 0 FOR column1  

Answer 2

You could write this in the trigger:

UPDATE T SET value1 =0 FROM   table_name T JOIN   INSERTED I ON     T.<id>=I.<id> WHERE  I.value1 is null 

INSERTED table which is accessible only within trigger will store the values that have inserted..