Beginning SQL

Chapter 1
The following SQL creates a table based on the earlier train timetable example:
CREATE TABLE Train_Times
(
start_location varchar(75),
destination varchar(75),
departs time,
arrives time
);
MS SQL Server doesn’t have the time data type, so you need to change the data type to datetime.
For Oracle, you need to change the data type to date rather than time, because Oracle’s date data
type stores both date and time.
For this and all of the examples in this book, load up the tool that came with your RDBMS that allows
SQL code to be written and run. You can find installation details in Appendix B at the end of the book.
Examining the code line by line, first you specify that you want to create a table called Train_Times:
CREATE TABLE Train_Times
Then, inside brackets, you specify the four fields that make up each record. For each record, you need to
identify the field name and data type:
(
)
start_location varchar(75),
destination varchar(75),
departs time,
arrives time
A comma must separate each field definition. Notice the SQL’s neat layout, with the CREATE TABLE
statement and each field definition on separate lines. Such layout is not compulsory. In fact, you could
cram the whole lot on one line. However, laying out the code on separate lines makes the code a lot
more readable and therefore easier to write and makes it easier for you to fix errors, or debug, if things go
wrong.
You can see that creating tables is pretty easy. There are more complexities with table creation than listed
here, and these are examined in Chapter 4. The next issue is how to alter tables. Say you want to add a
new field, delete an existing one, or perform other routine table maintenance. The good news is that SQL
allows you to perform all of these functions with one statement: ALTER TABLE.
Altering an Existing Table
26
The key to changing an existing table is the ALTER TABLE statement. This one statement allows you to
add and delete columns in an existing table. What the ANSI SQL standard ALTER TABLE statement
doesn’t let you do, however, are things like changing the data type of an existing column. However,
many RDBMSs have extended the ALTER TABLE statement and include their own way of changing column
definitions.