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Database Systems
dr Grzegorz Michalski
14 May 2014
Database Systems 1/26
General Functions
The following instructions work with any data type and pertain to
using nulls:
NVL (expr1, expr2)
NVL2 (expr1, expr2, expr3)
NULLIF(expr1, expr2)
COALESCE (expr1, expr2, . . . , exrn)
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NVL Function
Converts a null value to an actual value:
Data types that can be used are date, character, and number.
Data types must match:
NVL(commission pct,0)
NVL(hire date,’01=JAN-97’)
NVL(job id, ’No Job Yet’)
Database Systems 3/26
NVL, NVL2 examples
Example
SELECT last name, salary, NVL(commission pct, 0),
(salary*12 + (12*salary*NVL(commission pct, 0))
ANNUAL SALLARY,
FROM employees;
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NVL, NVL2 examples
Example
SELECT last name, salary, NVL(commission pct, 0),
(salary*12 + (12*salary*NVL(commission pct, 0))
ANNUAL SALLARY,
FROM employees;
Example
SELECT last name, salary, commission pct,
NVL2(commission pct, ’SAL and COMM’,’SAL’) income
FROM employees
WHERE department id IN (40,90);
Database Systems 4/26
NULLIF example
Example
SELECT first name, LENGTH(first name) "expr1",
last name, LENGTH(last name) "expr2"
NULLIF(LENGTH(first name), LENGTH(first name)) res
FROM employees;
Database Systems 5/26
Usign the COALESCE Function
1
The adventage of the COALESCE function over the NVL
function is that the COALESCE function can take multiple
alternate values.
2
If the first expression is not null, the COALESCE function
returns that expression; otherwise, it does a COALESCE of the
remaining expressions.
Example
SELECT last name, employee id,
COALESCE(TO CHAR(commission pct),
TO CHAR(manager id),
’No commision and no manager’) Info
FROM employees;
Database Systems 6/26
COALESCE exmaple
Example
SELECT last name, salary, commission pct,
COALESCE((salary + (commission pct*salary)),
salary+2000, salary) ”New Salary”)
FROM employees;
Database Systems 7/26
Conditional Expressions
1
2
Provide the use of the IF--THEN--ELSE logic within a SQL
statement.
Use a two methods:
CASE expression.
DECODE function.
CASE expression
Facilitates conditional inquiries by doing the work of an
IF--THEN--ELSE:
CASE expr WHEN comparison expr1 THEN return expr1
[WHEN comparison expr2 THEN return expr2
WHEN comparison exprn THEN return exprn
ELSE else expr]
END
Database Systems 8/26
Using the CASE expression
Example
SELECT last name, job id, salary,
CASE job id WHEN ’IT PROG’ THEN 2.50*salary
WHEN ’ST CLERK’ THEN 1.20*salary
WHEN ’SA REP’ THEN 1.25*salary
ELSE salary
END "Revised salary"
FROM employees;
Example
SELECT last name, job id, salary,
CASE WHEN salary < 5000 THEN ’Low’
WHEN salary < 10000 THEN ’Medium’
WHEN salary < 15000 THEN ’High’
ELSE ’Excellent’
END Salary
FROM employees;
Database Systems 9/26
DECODE Function
Facilitates conditional inquiries by doing work of a CASE expression
or an IF--THEN--ELSE statement
DECODE(col|expression, search1, result1
[, search2, result2,. . . ]
[, default])
Example
SELECT last name, job id, salary,
DECODE(job id, ’IT PROG’, 2.50*salary,
’ST CLERK’, 1.20*salary,
’SA REP’, 1.25*salary,
salary)
"Revised salary"
FROM employees;
Database Systems 10/26
DECODE example
Example
SELECT last name, salary,
DECODE( TRUNC( SALARY/2000, 0),
0, 0.00,
1, 0.09,
2, 0.20,
3, 0.30,
4, 0.40,
5, 0.42,
5, 0.44,
0.45)
"Tax Rate"
FROM employees;
Database Systems 11/26
Reporting Aggregated Data
Using the Group Functions
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What are group Functions?
group functions operate on sets of rows to give one result per
group.
Types of group functions
AVG
COUNT
MAX
MIN
STDDEV
VARIANCE
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Types of group functions
Function
AVG([DISTINCT|ALL] n)
COUNT([DISTINCT|ALL] expr)
MAX([DISTINCT|ALL] expr)
MIN([DISTINCT|ALL] expr)
STDDEV([DISTINCT|ALL] n)
SUM([DISTINCT|ALL] n)
VARIANCE([DISTINCT|ALL] n)
Description
Averange value of n,
ignoring nulls
Number of rows, where expr
evaluates to something
other than nulls
Maximum value of expr
ignoring nulls
Minimum value of expr,
ignoring nulls
Standard deviation of n,
ignoring nulls
Sum values of n, ignoring nulls
Variance of n, ignoring nulls
Database Systems 14/26
Group Functions: Syntax
SELECT group function(column), . . .
FROM table
[WHERE conditions];
Using the AVG and SUM Functions
You may use AVG and SUM for numeric data.
Example
SELECT AVG(salary), MAX(salary),
MIN(salary), SUM(salary)
FROM employees
WHERE job id LIKE ‘%REP%’;
Database Systems 15/26
Using the MIN and MAX Functions
You can use the MIN and MAX functions for numeric, character, and
date data types.
Example
SELECT MIN(hire date), MAX(hire date),
FROM employees
Example
SELECT MIN(last name), MAX(last name),
FROM employees
Database Systems 16/26
Using the COUNT Function
Example
COUNT(*) returns the number of rows in a table:
SELECT COUNT(*)
FROM employees
WHERE department id = 50
Example
COUNT(expr) returns the number of rows with non–null values for
expr:
SELECT COUNT(commission pct)
FROM employees
WHERE department id = 50
Database Systems 17/26
Using the DISTINCT Keyword
COUNT(DISTINCT expr) retuns the number of distinct
non–null values of expr
To display the number of distinct department values in the
EMPLOYEES table:
Example
SELECT COUNT(DISTINCT department id)
FROM employees;
Database Systems 18/26
Group Functions and Null Values
All group functions ignore null values in column,
The NVL function forces group functions to include null values.
Example
SELECT AVG(commision pct), AVG(NVL(commision pct, 0))
FROM employees;
Database Systems 19/26
Creating Groups of Data: GROUP BY Clause Syntax
You can divide rows in a table into smaller groups by using the
GROUP BY clause:
SELECT column, gropu function(column)
FROM table
[WHERE condition]
[GROUP BY group by expression]
[ORDER BY column]
Database Systems 20/26
Using the GROUP BY Clause
All the columns in the SELECT list that are not in group functions
must be in the GROUP BY clause.
Example
SELECT department id, AVG (salary)
FROM employees
GROUP BY department id;
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Using the GROUP BY Clause
The GROUP BY column does not have to be in the SELECT list.
In the ORDER BY clause you may use a group function.
Example
SELECT AVG(salary)
FROM employees
GROUP BY department id;
Example
SELECT department id, AVG(salary)
FROM employees
GROUP BY department id
ORDER BY AVG(salary)
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Using the GROUP BY Clause on Multiple Columns
Example
SELECT department id, job id, SUM(salary)
FROM emplyees
WHERE department id > 20
GROUP BY department id, job id
ORDER BY department id;
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Illegal Queries Using Group Functions
Any column or expression in the SELECT list that is not an
aggregate function must be in the GROUP BY clause.
If you use a aggregate function, you must use a GROUP BY
clause.
Example
SELECT department id, COUNT(last name)
FROM employees;
Example
SELECT department id, job id, COUNT(last name)
FROM employees
GROUP BY department id;
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Restricting Group Results with the HAVING Clause
When you use the HAVING clause, the Oracle server restricts
groups as follows:
Rows are grouped.
The group function is applied.
Groups matching the HAVING clause are displayed.
SELECT column, gropu function(column)
FROM table
[WHERE condition]
[GROUP BY group by expression]
[HAVING group condition]
[ORDER BY column]
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Using the HAVING clause
Example
SELECT department id, MAX(salary)
FROM employees
GROUP BY department id
HAVING MAX(salary) > 10000;
Example
SELECT job id, SUM(salary) payroll
FROM employees
WHERE job id NOT LIKE ’%REP%’
GROUP BY job id
HAVING SUM(salary) > 12500
ORDER BY SUM(salary);
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