School of Computing, Engineering and Information Sciences
University of Northumbria
Creating
SQL Tables
and using
Data Types
Aims:
To learn how to create tables in Oracle SQL, and how to use
Oracle SQL data types in the creation of these tables.
Outline of Session:
Given a simple database design, decide on suitable data types for
all the columns in all the tables of the database.
Create the tables using Oracle SQL.
Decide what columns must never be allowed to have data missing,
and re-create the tables accordingly.
From the results of the earlier exercise to determine what sort of
data would be needed to run a club or society, decide on 2 specific
relations of those discussed, and create SQL tables to represent
those relations.
School of Computing, Engineering and Information Sciences
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A SIMPLE DATABASE
Over the course of this module we will be developing a relational database to
represent the activities of a small company which runs various Projects and
which has its Employees organised into Departments. Thus we want to
create in the database a relation to hold all the related data about Projects,
another relation to hold all the related data about Employees, and a third
relation to hold all the related data about Departments.
Note that SQL uses the term ‘tables’ for ‘relations’, ‘row’ for ‘tuple’ and
‘column’ for ‘attribute’, although strictly speaking, the SQL terms do not
always mean exactly the same as the relational terms. When differences
arise, they will be mentioned.
So to create a database, we will actually need to create tables and specify the
columns that the tables contain.
Consider the information below, which describes what data we want to hold in
the Employees, Departments and Projects tables respectively :
Table Name
EMP
DEPT
PROJ
Column Name
EMP_NO
Description
Data type
A two-character code, not
necessarily both digits.
EMP_NAME
The Employee’s surname: up to
10 letters.
SALARY
Whole number of pounds.
MARITAL_STATUS A one-character coded value.
DEPT_NO
A two-character code, not
necessarily both digits.
DEPT_NAME
Name of this Department: up to
10 letters.
MANAGER_NO
The Employee Number of the
Manager of this Department.
BUDGET
Whole number of pounds.
PROJ_NO
A two-character code, not
necessarily both digits.
START_DATE
The date when the Project
started.
DEADLINE
Date when the Project must be
completed.
Exercise: Fill in the right-hand column above with suitable SQL data types.
Decide in principle what kind of data should be stored in each column. Then decide what
kind of Oracle SQL data type serves your purpose. (You will need to look up available
types in an Oracle SQL reference).
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SQL SYNTAX
As with other programming languages, there is a precise set of rules
governing the syntax (grammar) of SQL. We will present examples of SQL
syntax as and when they are needed in this and succeeding exercises. As
each SQL construct is introduced, a box containing details of its makeup will
be displayed, according to the following conventions:

CAPITALS denote reserved words (i.e. part of the vocabulary of SQL
with particular meaning within the language).

<Angled brackets> contain the name of a syntactic construct that is
described elsewhere.

underlined italics denote the name of some object within the
database (e.g. a table or column name), or a numeric or character-string
value as appropriate.

Character strings or patterns must be enclosed in ‘single quotation
marks’. A pattern is similar to a character string but may contain the
wild-card characters % (percent) and _ (underscore). Character strings
are also known as literals or string constants.

[square brackets] denote an optional item.

{<option 1> | <option 2> | <option 3>} vertical bars separate
alternative items enclosed by curly brackets (braces). Only one of the
alternatives is to be used. The braces may be omitted where the
meaning is clear.

[ etc.] indicates that the construct immediately before the square
brackets may, optionally, be repeated any number of times. If a
separator is required it will be shown like this [ , etc.]
When devising Data Names (e.g. names for tables or columns) remember
they must start with a letter, the rest of the name being optionally a mixture
of letters, digits and the – (hyphen) and _ (underscore) characters.
SQL statements run continuously until terminated by a semi-colon, but it can
be useful to break a statement into several lines to enhance readability.
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CREATING TABLES IN ORACLE
SQL provides a CREATE TABLE statement for setting up database tables.
As a minimum, this requires us to give the table a name and then to specify
for each column of the table:

its name;

the data type of the data values that it is to contain.
In its simplest form, the CREATE TABLE statement has the syntax
CREATE TABLE table-name ( <column definition> [ , etc.] ) ;
Following the reserved words CREATE TABLE, the actual name of the table
must be supplied. This is followed by a pair of (round) brackets containing
one or more instances, separated by commas, of a column definition. The
statement ends (as do other SQL statements) with a semi-colon.
A column definition, in its simplest form, has the syntax
column-name <data type>
In plain English, what does this syntax definition mean?
The most commonly used data types in Oracle SQL are:

INTEGER, i.e. a whole number (which may have a positive or
negative value)

DECIMAL. This requires you to specify firstly the precision
(number of significant figures) and then the number of decimal places
– e.g. DECIMAL (7, 2)

CHAR, i.e. a string of characters. It is necessary to specify the
number of characters to be held, e.g. CHAR(10). If less than 10
characters (in this example) are used, then Oracle will add sufficient
‘space’ characters onto the end of the string such that there is a total of
10 characters.

VARCHAR21, i.e. a variable length string of characters. It is
necessary to specify the maximum number of characters that could be
held, e.g. VARCHAR2(10). If less than 10 characters (in this example)
are used, then no extra ‘space’ characters’ are added.

DATE, which will be formatted according to a standard convention.
The ‘2’ in ‘VARCHAR2’ indicates that it is Oracle’s second version of the variable length character
type. It has no relevance to how many characters may or should be held in the character string.
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You should now be able to create all three tables, EMP, DEPT and PROJ.
To get you started, here is the SQL code for creating the EMP table. You should enter the
following code into Oracle, and then try the same procedure for the DEPT and PROJ
tables. (Remember, you do not need to enter the line numbers!)
SQL> CREATE TABLE EMP(
2
EMP_NO CHAR(2),
3
EMP_NAME VARCHAR2(10),
4
SALARY INTEGER,
5
MARITAL_STATUS CHAR(1)
6
);
EDITING SQL STATEMENTS
During the course of entering these 3 tables into your database, make a point
of learning how to edit SQL statements, both by means of the copy, cut and
paste facilities provided by the Oracle DBMS interface, and by means of the
editor built into the interface. (Note that this interface is proprietary to Oracle,
and therefore may be quite different to those of other SQL DBMSs).
CHECKING YOUR WORK
Assuming that your CREATE TABLE statements were syntactically correct,
Oracle will have replied to each with the message “Table created”. However,
this is not really sufficient; you need to be able to check that the column
names and data types are as you intended. It is possible to do this by means
of the DESCRIBE command. This is not an SQL statement but a command
which is specific to Oracle, and it does not need a semi-colon after it. Here is
an example of its use :
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SQL> DESCRIBE EMP
Name
Null?
------------------------------- -------EMP_NO
EMP_NAME
SALARY
MARITAL_STATUS
Type
---CHAR(2)
VARCHAR2(10)
NUMBER(10)
CHAR(1)
Exercise: Some of the data types shown in the output from DESCRIBE will differ somewhat
from what you originally typed in. What examples of this can you find?
This is because the data types used by Oracle differ somewhat from those
stipulated by the SQL Standard. Nevertheless Oracle does recognise the
Standard data types, and these are what we are recommending you to use.
If at any time you want to check what tables you have in your database, you
can use the following SQL statement :
SELECT * FROM cat ;
This will give you a list, by table name, of all the tables in your database.
‘cat’ is short for ‘catalog’ (note the American spelling). Every DBMS uses a
database catalog(ue) or data dictionary or meta database – the terms vary
depending on the prevailing DBMS vendor and computing fashion – to hold
data about the database. (SQL normally uses the term ‘catalog’). All the
above query is actually doing is retrieving the contents of the table ‘catalog’,
wherein is stored the list of table names that we want. ‘catalog’ is one of the
tables in the database catalog(ue) as a whole.
The DESCRIBE command works by executing a more sophisticated retrieval
on parts of the overall database catalog(ue).
DECIDING ON NULLS
The tables created so far permit data to be missing from rows in every column
of every table; i.e. allow NULLs to appear there instead. In practice, this
would often be unacceptable.
Therefore review all the columns in all the tables, and decide which columns
must always have a value in every row of the table; in other words, for these
columns there can never be any exceptions, and every row must always have
a value of the required type in that column.
For example, in the PROJ table, it is reasonable to say that every row must
have a START_DATE value, since we will not put a row in the table to
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represent a real-world project until we know when it will start (and hence that
it definitely will start). However DEADLINE values may sometimes be missing
because realistically we are not always certain when a project should finish
until some considerable way through it; hence the DEADLINE column may
have NULLs as well as deadline dates.
Exercise: For each column in the 3 tables, decide whether NULLs are permissible or not.
For each column, enter your conclusion in the right-hand page margin opposite the
column’s name in the tabular design summary on page 2; then summarise your rationale
for each column below :
Having decided on which columns must never contain NULLs, you will need
to amend the SQL table definitions to accomplish this.
There are two ways to do this :
1. Drop a table (i.e. eliminate or delete it from the database) and recreate it to its new specification.
2. Alter the table appropriately.
In principle, the second way is simpler, and indeed may be the only
practicable way if data is stored in the table. However it uses the SQL
ALTER TABLE command, which can be complex to use until you have
mastered all the ramifications of specifying and creating tables. Therefore we
postpone consideration of the ALTER TABLE command until later in the
course, and use the first method instead; our tables contain no data yet, so
this is not a problem.
So get rid of all 3 tables from the database.
Use the SQL command
DROP TABLE table-name ;
where table-name is the name of the table to be dropped or got rid of.
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Then re-create the 3 tables with the appropriate NON NULL columns.
The syntax to be used when specifying that a column cannot contain NULLs
is simply to put the phrase NOT NULL after the column’s data type. Thus
the formal syntax of a column specification within a CREATE TABLE
statement is :
column-name <data type> [ NOT NULL ]
Remember ‘[ ‘and ‘]’ denote an option in the syntax – do not enter them.
FURTHER EXERCISES
Refer back to the previous exercise where you developed some relations that
would form a useful database for a club or society. Select at least 2 of those
relations, and :
1. Decide on a name for each relation.
2. For each attribute in each relation, decide on an attribute name and
data type, and whether it must always hold a data value or whether
missing data would be allowed.
3. Derive an SQL CREATE TABLE statement to create the SQL
equivalent of each relation, and enter it into your database.
4. Check in the Oracle SQL catalog(ue) whether you have created the
tables correctly or not. If they contain any errors, drop them and recreate them.
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