A default specifies a value that your DBMS assigns to a column if you omit a value for the column when inserting a row; see “Inserting Rows with INSERT” in Chapter 9. When you’re defining a default constraint, some important considerations are:
A default constraint always is a column constraint and not a table constraint; see “Understanding Constraints” earlier in this chapter.
You define a default constraint by using the keyword DEFAULT in a CREATE TABLE column definition.
A default value can be any expression that evaluates to a constant.
The default must have the same data type or must be implicitly convertible to the same type as its column; see “Converting Data Types with CAST()” in Chapter 5.
The column ...