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, they're not redundant.
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.
来源：https://stackoverflow.com/questions/11862188/sql-column-definition-default-value-and-not-null-redundant