Structured Query Language
Basic Queries in SQL
 SQL has one basic statement for
retrieving information from a
database
 The Select Statement
Basic SELECT Statement
In its simplest form, a SELECT
statement must include the following:
A SELECT clause, which specifies the
columns to be displayed
A FROM clause, which specifies the table
containing the columns listed in the SELECT
clause
Selecting All Columns, All Rows
 You can display all columns of data in
a table by following the SELECT
keyword with an asterisk (*).
 Example:
 Select * from department;
Selecting Specific Columns, All
Rows
 You can use the SELECT statement to
display specific columns of the table
by specifying the column names,
separated by commas.
 Example:
 Select fname , lname from employee;
ARITHMETIC EXPRESSIONS
Arithmetic operators such as +, -, *,
/ may be used in SQL statements.
ex.
SELECT fname, salary+100
FROM employee;
Duplicate Rows
 The default display of queries is all
rows, including duplicate rows
 To eliminate dublicate rows we use
DISTINCT keyword in select clause
Duplicate rows
 Example :
 Select distinct salary
from employee
Column Alias
Column aliases exist to help organizing output.
The syntax:
SELECT column AS column_alias FROM table
Ex. How to use alias
SELECT Lname As employee
FROM employee;
Or: SELECT Lname employee
FROM employee;
Or: SELECT Lname “Employee Record”
FROM employee;
Concatenation and Literals
 For string data types, the concatenate
operator || can be used in a query to
append two string values.
 Literals refers to a fixed data value such
as string, number or date. It will be
viewed one time in each row.
Example

Select SSN, Fname || ' ' || Minit ||
' ' || Lname From Employee;
Ordering of Query Results
 SQL allows the user to order the rows in the result of
a query by the values of one or more attributes, using
the ORDER BY clause.
 The default order is in ascending order of values. We
can specify the keyword DESC if we want to see the
result in a descending order of values. The keyword
ASC can be used to specify ascending order explicitly.
 if we want descending order on DNAME and ascending
order on LNAME, FNAME the order by clause will be:
ORDER BY DNAME DESC, LNAME ASC, FNAME ASC
Limiting Row Selection



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The queries we wrote so far display all the rows from a table.
When you select information from the database sometimes it is
desirable to limit the rows that you select. For example if there
were 5000 rows of data in the employee table and you wanted to
look at the details of only one employee then you would not want
to have to select all 5000 rows. You would only want to retrieve
the details of the employee you were interested in. You can reduce
the number of rows selected by use of the where clause.
The where clause is part of the select statement and is optional.
The where clause limits the number of rows retrieved according to
some criteria
The where clause, if used, must follow the FROM clause. It
contains a condition, which rows must meet in order to be
displayed.
Select columns
From table
Where certain_condition;
Example
 For example if you want to display a
list of all those employees who are
working in department 4, the SQL
statement will be
Select ssn, fname, dno
From employee
Where dno = 4;
Comparison Operators



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=, < , <=,>, >= and <>
NOT
AND
OR
 AND and OR join two or more conditions in a WHERE
clause. You can also combine AND and OR.
 IN (value1,value2,..)

 The IN operator may be used if you know the exact
value you want to return for at least one of the
columns.
BETWEEN lowest_value AND highest_value
 To select range of data.
 LIKE
 % zero or more
 _ one character
 Retrieve the birthdate and address of
the employee whose name is ‘John B.
Smith’
 SELECT BDATE,ADDRESS
FROM EMPLOYEE
WHERE FNAME=‘John’ AND MINIT=‘B’
AND LNAME=‘Smith’;
 Retrieve the name(s)of the
employee(s) whose salary not equal
38000
 SELECT fname,lname
FROM EMPLOYEE
WHERE salary <> 38000;
 Retrieve all employees in department 5
whose salary is between $30,000 and
$40,000.
SELECT
*
FROM
WHERE
EMPLOYEE
(SALARY BETWEEN 30000 AND 40000) AND
DNO = 5;
Selecting Rows from a Set of
Values Using IN Operator
 The IN operator tests for values in a specified list.
 For example, if you want to list employees who are
working in either dno 1, or dno 2 or dno 4, then you
may use the following SQL statement
Select *
From employee
Where dno=1 OR dno = 2 OR dno = 4;
 The above query can be rewritten using IN operator
as
Select *
From employee
Where DNO IN (1, 3, 4);
 Retrieve the social security numbers
of all employees who work on project
number 1, 2, or 3.
SELECT
FROM
WHERE
DISTINCT ESSN
WORKS_ON
PNO IN (1, 2, 3)
Substring Comparison
 SQL allows comparison conditions on only
parts of a character string , using LIKE
comparison operator.
 Partial strings are specified by using two
reserved character:
 % : replaces an arbitrary number of
characters.
 _ : replaces a single character.
Example of substring comparisons
 'ABC%': All strings that start with 'ABC'. For
example, 'ABCD' and 'ABCABC' would both
satisfy the condition.
 '%XYZ': All strings that end with 'XYZ'. For
example, 'WXYZ' and 'ZZXYZ' would both
satisfy the condition.
 '%AN%': All strings that contain the pattern
'AN' anywhere. For example, 'LOS ANGELES'
and 'SAN FRANCISCO' would both satisfy the
condition.
 ‘C _ _‘ : Any three characters long string that
starts with C . For example , ‘Car’ and
‘Cat’ would both satisfy the condition.
 Retrieve all employees whose
address is Huston, Texas
SELECT
FROM
WHERE
FNAME, LNAME
EMPLOYEE
ADDRESS LIKE
'%Houston,TX%';
 Retrieve all employees who were
born during the 1950s.
SELECT
FROM
WHERE
FNAME, LNAME
EMPLOYEE
BDATE LIKE ’_ _ _ _ _ _ _ _ _5_';
NULLS IN SQL QUERIES
 SQL allows queries that check if a value is NULL
(missing or undefined or not applicable)
 SQL uses IS or IS NOT to compare NULLs because it
considers each NULL value distinct from other NULL
values, so equality comparison is not appropriate .
 Retrieve the names of all employees who do not have
supervisors.
SELECT
FNAME, LNAME
FROM
EMPLOYEE
WHERE
SUPERSSN IS NULL
Note: If a join condition is specified, tuples with NULL
values for the join attributes are not included in the
result