SQL Column definition : default value and not null

问题:

I've seen many times the following syntax which defines a column in a create/alter DDL statement:

ALTER TABLE tbl ADD COLUMN col VARCHAR(20) NOT NULL DEFAULT "MyDefault"

The question is: since a default value is specified, is it necessary to also specify that the column should not accept NULLs ? In other words, doesn't DEFAULT render NOT NULL redundant ?

回答1:

DEFAULT is the value that will be inserted in the absence of an explicit value in an insert / update statement. Lets assume, your DDL did not have the NOT NULL constraint:

ALTER TABLE tbl ADD COLUMN col VARCHAR(20) DEFAULT "MyDefault"

Then you could issue these statements

-- 1. This will insert "MyDefault" into tbl.col
INSERT INTO tbl (A, B) VALUES (NULL, NULL);

-- 2. This will insert "MyDefault" into tbl.col
INSERT INTO tbl (A, B, col) VALUES (NULL, NULL, DEFAULT);

-- 3. This will insert "MyDefault" into tbl.col
INSERT INTO tbl (A, B, col) DEFAULT VALUES;

-- 4. This will insert NULL into tbl.col
INSERT INTO tbl (A, B, col) VALUES (NULL, NULL, NULL);

Alternatively, you can also use DEFAULT in UPDATE statements, according to the SQL-1992 standard:

-- 5. This will update "MyDefault" into tbl.col
UPDATE tbl SET col = DEFAULT;

-- 6. This will update NULL into tbl.col
UPDATE tbl SET col = NULL;

Note, not all databases support all of these SQL standard syntaxes. Adding the NOT NULL constraint will cause an error with statements 4, 6, while 1-3, 5 are still valid statements. So to answer your question:

No, NOT NULL and DEFAULT are not redundant. In particular, NOT NULL can have a tremendous impact on query performance as explained in this blog post here

回答2:

Even with a default value, you can always override the column data with null.

The NOT NULL restriction won't let you update that row after it was created with null value

回答3:

My SQL teacher said that if you specify both a DEFAULT value and NOT NULLor NULL, DEFAULT should always be expressed before NOT NULL or NULL.

Like this:

ALTER TABLE tbl ADD COLUMN col VARCHAR(20) DEFAULT "MyDefault" NOT NULL

ALTER TABLE tbl ADD COLUMN col VARCHAR(20) DEFAULT "MyDefault" NULL

回答4:

I would say not.

If the column does accept null values, then there's nothing to stop you inserting a null value into the field, as far as I'm aware, the default value only applies on creation of a new rule.

With not null set, then you can't insert a null value into the field as it'll throw an error.

Think of it as a fail safe mechanism to prevent nulls.