An Introduction To SQL - Part 1(Special thanks to Geoff Leese)

Select Statements The Where Clause Manipulating data using the EMP and DEPT tables

using Oracle/MySQL so that you get a feel for the SQL language.

Creating tables, populating them with data and manipulating the data.

The tutorials will allow you to work through a series of questions to reinforce the lecture material.

You may find it useful to keep your lecture material with you during the tutorials.

Data Manipulation Language (DML)◦ Insert allows you to enter new rows◦ Update allows you to change existing rows ◦ Delete remove unwanted rows

Select is used to retrieve data from the database Data Description Language (DDL)

◦ Create create objects◦ Alter structurally change objects◦ Drop drop objects within the database(Objects are tables, views, indexes etc.)

Data Control Language (DCL)◦ Grant specify access rights on objects◦ Revoke remove access rights

These statements are NOT case-sensitiveEach of the statements should be terminated with asemicolon ‘;’ at the end of the last line, OR a slash can be used on the following line.

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO

7839 KING PRESIDENT - 11/17/1981 5000 - 10

7698 BLAKE MANAGER 7839 05/01/1981 2850 - 30

7782 CLARK MANAGER 7839 06/09/1981 2450 - 10

7566 JONES MANAGER 7839 04/02/1981 2975 - 20

7788 SCOTT ANALYST 7566 12/09/1982 3000 - 20

7902 FORD ANALYST 7566 12/03/1981 3000 - 20

7369 SMITH CLERK 7902 12/17/1980 800 - 20

7499 ALLEN SALESMAN 7698 02/20/1981 1600 300 30

7521 WARD SALESMAN 7698 02/22/1981 1250 500 30

7654 MARTIN SALESMAN 7698 09/28/1981 1250 1400 30

7844 TURNER SALESMAN 7698 09/08/1981 1500 0 30

7876 ADAMS CLERK 7788 01/12/1983 1100 - 20

7900 JAMES CLERK 7698 12/03/1981 950 - 30

7934 MILLER MANAGER 7782 01/23/1982 1300 - 10

DEPTNO DNAME LOC

10 ACCOUNTING NEW YORK20 RESEARCH DALLAS30 SALES CHICAGO40 OPERATIONS BOSTON50 HR LONDON

This is the most commonly used statement Used to query database information The informal syntax is a follows:

SELECT column_name(s)FROM table_name(s)WHERE constraint(s);

‘*’ asks for all the columns to be returned.

The top line ‘SELECT’ describes the column name(s) from which you are attempting to build the query.

The second line ‘FROM’ lists the tables or relations that contain the data necessary to service the query

The third line, the ‘WHERE’ constraint is optional. The WHERE clause allows you to specify the tuples (rows) to be returned.

The basic statement: If we wish to display all of the tuples (rows) of a given relation (i.e.

table):

SELECT *FROM EMP; (; end of statement to be executed)

◦ We can constrain this by requesting only those employees with the name ‘Allen’

SELECT *FROM EMPWHERE ENAME = 'ALLEN'; (this is the constraint)

◦ 'ALLEN' is enclosed in quotation marks as we are searching on a column data type which is a string

If we want to search on numeric columns:

SELECT *FROM DEPTWHERE DEPTNO = 10;

SELECT *FROM EMPWHERE EMPNO = 7654;

This clause is used to restrict the output to show a subset of the rows from a table.

It must be placed after the FROM clause The WHERE clause has the following format: WHERE Operators may be:

◦ Comparative Operators◦ = != < > <= >= <> and◦ Logical Operators

AND, OR, NOT

SELECT *FROM EMPWHERE EMPNO = 7521 AND ENAME = 'WARD';

If we said:SELECT *FROM EMPWHERE EMPNO = 7369 AND ENAME = 'WARD';We would return an empty table as there are no tuples

(rows)that satisfy both of these constraints.

if we said:SELECT *FROM EMPWHERE EMPNO = 7876 OR ENAME = 'KING';Two tuples would be returned as these would satisfy one orother of the constraints.

You can retrieve attribute values by specifying columns: The command is

SELECT COLUMN_NAME (S)FROM TABLE_NAME (S);

SELECT ENAMEFROM EMP;

Will return one column, and the values from the ename column (14 rows).

SELECT EMPNO,ENAME,JOBFROM EMP;

Will return three columns, in that order (14 rows)

Row and Columns constraints can be set as follows:SELECT EMPNO, ENAMEFROM EMPWHERE SAL > 2000;

Returns only those columns empno and ename from the emp table and only those rows with a balance greater than 2000.

SELECT EMPNO, ENAME, JOB, SALFROM EMPWHERE SAL BETWEEN 1500 AND 3000;

Displays all those employees who earn between 1500 and 3000 (four columns and eight rows in this case)

If we want to search for employees whose name begins with the letter ‘A’We can say:

SELECT EMPNO, ENAME, JOB

FROM EMPWHERE ENAME LIKE 'A%';

This will display two tuples (rows) i.e. all those employees whose names begin with A and three columns.

The ‘%’ is a wildcard operator.

You can use the ‘NOT IN’ operator as follows:SELECT ENAME, JOB, SALFROM EMPWHERE JOB NOT IN ('SALESMAN', 'MANAGER');

This will return those employees who are not salesmen or managers.

You can also use the operator !=

If you want to find all the clerks in department 30 who earn less than $1000 you can say:

SELECT ENAME,JOB, SALFROM EMPWHERE DEPTNO = 30AND JOB = 'CLERK'AND SAL < 1000;This will return 1 row for James, a clerk who earns $950.

MySQL provides you with the capability to gather and manipulate data over several tables using the JOIN feature.

A join is one of the fundamental operations in relational algebra.

The basic Structure is:SELECT table1.column1, table1.column2, table2.column1 FROM table1, table2, tablenWHERE join criteria;

◦ We want to know where ALLEN works. The EMP table does not hold the location details but the DEPT table does.

SELECT ename, locFROM EMP,DEPTWHERE EMP.deptno = DEPT.deptnoAND ename = 'ALLEN';

We need to distinguish between the deptno in the two tables therefore they are prefixed with the table name.Here we have an equi-join in the WHERE clause.

SELECT d.*, ename, job from EMP e, DEPT dWHERE e.deptno = d.deptno AND (e.deptno = 40 ore.deptno=30) ORDER BY e.deptno; The alias d.* will retrieve all the column names in the DEPT Table.

We use aliases to identify which column name belongs towhich table as some column names will exist in more thanone table for example, deptno.

Normally when tables are joined we use primary/foreign keys which are generally defined as NOT NULL, however to join non key elements we may use LEFT JOIN

◦ i.e Department 40 in table DEPT has no matching employees in table EMP so normal join queries would not retrieve department 40

SELECT dept.deptno, dname, job, enameFROM dept LEFT JOIN empon dept.deptno = emp.deptnoWHERE dept.deptno IN (30, 40)ORDER BY dept.deptno;

A view is a type of window that allows you to view selected portions of data.

Advantages: Security: a secure table may contain sensitive or confidential

information that prevents it from general circulation. You can deny access to the emp table itself but instead provide access to a VIEW without the salary or commission details.

A view can be constructed on a need to know basis. Convenience: instead of using complex queries to get a

perspective on your data, you can create a view which allows you to obtain the same information with a simple query.

No additional memory is required for a view as the view is created from the base tables every time the view is called.CREATE VIEW view_name AS(SELECT columns FROM table_name WHERE row_selector);

CREATE VIEW EMP10 ASSELECT EMPNO, ENAME,JOBFROM EMPWHERE DEPTNO = 10;

A message will be displayed ‘view created’ you can

check your view by: SELECT * FROM EMP10;

You can add, delete or update data in a view.UPDATE EMP10SET job = 'MANAGER'Where ename = 'MILLER';

Delete, rename or drop a view.DELETE FROM EMP10WHERE ename = 'WILSON';

RENAME EMP10 to EMP11;

DROP VIEW EMP11;

When one of the conditions in a WHERE clause is a query itself it is known as a nested sub-query. It is used for step-wise processing.

SELECT ename, jobFROM EMPWHERE job = (SELECT job FROM EMP WHEREename = 'JONES'); The sub query is enclosed in brackets. The values compared across the outer query and the

subquery must be of the same datatype. Nested query is performed in step 1 and its result is used in

step 2 with the outer query.

Example: Find the person who earns the highest salary:◦ Step 1: find the maximum salary◦ Step 2: Find the person whose salary is equal to the maximum

salary

SELECT ename, salFROM EMPWHERE sal = (SELECT MAX(sal) FROM EMP);

More DML◦ – additional SELECT clauses◦ Insert, Alter and Delete

DDL – CREATing, ALTERing and DROPping objects

Rolland, F.D (1997), Essence of Databases (Essence of Computing), Prentice-Hall. ISBN 0137278276 

Date C.J. (2000); An Introduction to Database Systems, Addison Wesley, (or any earlier editions)

Patrick J.P. (2002), SQL Fundamentals (2nd edition), Prentice Hall, ISBN 0130669474

SQL at w3schools (click to follow the link)