A Guide to SQL, Seventh Edition

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


A Guide to SQL, Seventh Edition

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