Database Reference
In-Depth Information
▼
Analysis
This statement creates the
orderitems
table that contains the individual
items that make up an order. (The order itself is stored in the
orders
table.)
The
quantity
column contains the quantity for each item in an order. In
this example, adding the text
DEFAULT 1
to the column description instructs
MariaDB to use a quantity of
1
if no quantity is specified.
Caution
Functions Are Not Allowed Unlike most DBMSs, MariaDB (like MySQL) does not allow
the use of functions as
DEFAULT
values; only constants are supported.
Tip
Using
DEFAULT
Instead of
NULL
Values Many database developers use
DEFAULT
values instead of
NULL
columns, especially in columns that will be used in calculations
or data groupings.
You may have noticed that the
CREATE TABLE
statements used thus far all
ended with a
ENGINE=Aria
statement.
Like every other DBMS, MariaDB has an internal engine that actually man-
ages and manipulates data. When you
CREATE TABLE
that engine is used to
actually create the tables, and when you
SELECT
or perform any other database
processing, the engine is used internally to process your request. And for the
most part, the engine is buried within the DBMS and you need not pay much
attention to it.
But unlike most other DBMSs, MariaDB does not come with a single engine.
Rather, it ships with several different engines (the ones that come with MySQL
as well as additional ones), all buried within the server, and all capable of exe-
cuting commands like
CREATE TABLE
and
SELECT
.
So why bother shipping multiple engines? Because each has different capabili-
ties and features, and being able to pick the right engine for a job gives you
unprecedented power and flexibility.
Of course, you are free to totally ignore database engines. If you omit the
ENGINE=
statement, the default engine will be used, and most of your SQL
statements will work as is. But not all, and that is why this is important (and
why two different engines are used in the sample tables used in this topic).
