05. Single Table Query - Sistem Basis Data

A Guide to SQL, Seventh Edition

Objectives
 Retrieve data from a database using SQL
commands
 Use compound conditions
 Use computed columns
 Use the SQL LIKE operator
 Use the SQL IN operator
 Sort data using the ORDER BY clause

Objectives
 Sort data using multiple keys and in ascending and
descending order
 Use SQL aggregate functions
 Use subqueries
 Group data using the GROUP BY clause
 Select individual groups of data using the HAVING
clause
 Retrieve columns with null values

Constructing Simple Queries
 Important feature of DBMS is ability to answer a
wide variety of questions about the data
 A query is a question represented in a way that
the DBMS can understand
 Use the SELECT command to query a database

Constructing Simple Queries
 SELECT-FROM-WHERE is the basic form of the
command
 SELECT clause is the list of columns to include in
query results
 FROM clause is the name of the table with the
data being queried
 WHERE clause is optional, listing any conditions
to apply to the data

Retrieving Certain Columns and Rows
 A command can retrieve specified columns and all
rows
 List the number, name and balance of all
customers
 No WHERE clause is needed, because all
customers are requested

Retrieving All Columns and Rows
 Use an asterisk (*) to indicate all columns in the
SELECT clause
 Results will list all columns in the order in the
description when the table was created
 List columns in SELECT clause to present
columns in a different order

Using a WHERE Clause
 WHERE clause is used to retrieve rows that
satisfy some condition
 What is the name of customer number 148?
 A simple condition form: column name,
comparison operator, and then either a column
name or a value

Using a WHERE Clause
 Character values in SQL are case sensitive
 “Grove” is different than “grove”
 Simple conditions can compare columns
 WHERE BALANCE > CREDIT_LIMIT

Using Compound Conditions
 Compound conditions connect two or more
simple conditions with AND, OR, and NOT
operators
 AND operator shows results that all simple
conditions are true
 OR operator shows results that any simple
condition is true
 NOT operator reverses the truth of the original
condition

Using the BETWEEN Operator
 Not an essential feature in SQL
 Same results can be obtained without it
 Does make certain SELECT commands simpler to
construct
 BETWEEN operator is inclusive
 When using BETWEEN 2000 and 5000, values of
2000 or 5000 would be true

Using Computed Columns
 Computed column does not exist in the database
but is computed using data in existing columns
 Computations can involve arithmetic operators
 + for addition
 - for subtraction
 * for multiplication
 / for division

Using the LIKE Operator
 Used to retrieve data where there may not be an
exact match using wildcards
 LIKE %Central% will retrieve data with those
characters
 “3829 Central” or “Centralia”
 Underscore (_) represents any single character
 “T_M” for TIM or TOM or T3M

Using the IN Operator
 IN operator allows for concise phrasing of certain
conditions

Sorting
 ORDER BY clause to list data in a specific order
 Column on which data is to be sorted is the sort
key
 Use ORDER BY clause followed by sort key
 Rows are sorted in ascending order unless
another order is specified

Additional Sorting Options
 Possible to sort data by more than one key
 Major sort key and minor sort key
 List sort keys in order of importance in the
ORDER BY clause
 For descending order sort, use DESC

Using Functions
 Aggregate functions calculate sums, averages,
counts, minimum and maximum values to groups
of rows

Using the COUNT Function
 Counts the number of rows in a table
 Use of an asterisk allowed to represent any
column

Using the SUM Function
 Used to calculate totals of columns
 Column to be summed must be specified and
must be numeric
 AVG, MAX, and MIN functions are similar,
resulting in different statistics
 Null values are ignored and not used in these
calculations

Using the DISTINCT Operator
 Used to ensure uniqueness in the data results

Nesting Queries
 Some queries will take two or more steps to obtain
desired results
 A subquery is an inner query placed inside
another query
 Outer query can use results of the subquery to
find its results

Grouping
 Grouping creates groups of rows that share
common characteristics
 Calculations in the SELECT command are
performed for entire groups
 Data can be GROUPED BY a particular column,
such as REP_NUM and then the statistics are
calculated
 One line of output is produced for each group

Using a HAVING Clause
 The HAVING clause is used to restrict groups that
will be included

Nulls
 Sometimes a condition involves a column that can
be null

Summary
 Create queries that retrieve data from single tables
using SELECT commands
 Comparison operators: =, >,=>,<,=<, or <>, or !=
 Compound conditions using AND,OR, and NOT
 Use the BETWEEN operator
 Use the LIKE operator

Summary
 Use the IN operator
 Use and ORDER BY clause to sort data
 Process aggregate functions with COUNT,
SUM,AVG,MAX, and MIN
 Use the DISTINCT operator and subqueries
 Use GROUP BY, HAVING and IS NULL

05. Single Table Query - Sistem Basis Data

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05. Single Table Query - Sistem Basis Data