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..

Answer 3

在INSERTED值上使用ISNULL

 SELECT ISNULL(INSERTED,0) FROM INSERTED