Basic SQL SELECT Statements

Introduction to Oracle9i: SQL 1

Basic SQL SELECT Statements

Introduction to Oracle9i: SQL 2

Chapter Objectives

• Distinguish between an RDBMS and an ORDBMS

• Identify keywords, mandatory clauses, and optional clauses in a SELECT statement

• Select and view all columns of a table• Select and view one column of a table

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Chapter Objectives

• Display multiple columns of a table • Use a column alias to clarify the contents of

a particular column • Perform basic arithmetic operations in the

SELECT clause

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Chapter Objectives

• Remove duplicate lists, using either the DISTINCT or UNIQUE keyword

• Combine fields, literals, and other data• Format output

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Relational Database Management System (RDBMS)

An RDBMS is the software program used to create the database and it allows you to enter, manipulate, and retrieve data

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Object Relational Database Management System (ORDBMS)

Same as an RDBMS except it can be used to reference objects such as maps and object fields

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SELECT Statement Syntax

• SELECT statements are used to retrieve data from the database

• Syntax gives the basic structure, or rules, for a command

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SELECT Statement Syntax

Optional clauses and keywords are shown in brackets

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SELECT Statement Syntax

• SELECT and FROM clauses are required• SELECT clause identifies column(s)• FROM clause identifies table(s)• Each clause begins with a keyword

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Selecting All Data in a Table

Substitute an asterisk for the column names in a SELECT clause

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Selecting One Column from a Table

Enter column name in SELECT clause

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Selecting Multiple Columns from a Table

Separate column names with a comma

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Operations Within the SELECT Statement

• Column alias can be used for column headings

• Perform arithmetic operations• Suppress duplicates• Concatenate data

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Column Alias

• List after column heading• AS keyword is optional• Enclose in double quotation marks:

– If it contains blank space(s)– If it contains special symbol(s)– To retain case

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Column Alias Example

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Arithmetic Operations

• Executed left to right• Multiplication and division are solved first• Addition and subtraction are solved last• Override order with parentheses

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Example Arithmetic Operation with Column Alias

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Suppressing Duplicates

Enter DISTINCT or UNIQUE after SELECT keyword

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Concatenation

• Can combine data with string literal • Use concatenation operator, ||• Allows use of column alias

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Concatenation Example

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Purpose of Joins

• Joins are used to link tables and reconstruct data in a relational database

• Joins can be created through:– Conditions in a WHERE clause– Use of JOIN keywords in FROM clause

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Cartesian Join

• Created by omitting joining condition in the WHERE clause or through CROSS JOIN keywords in the FROM clause

• Results in every possible row combination (m * n)

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Cartesian Join Example:Omitted Condition

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Cartesian Join Example:CROSS JOIN Keywords

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Equality Join

• Links rows through equivalent data that exists in both tables

• Created by:– Creating equivalency condition in the WHERE

clause– Using NATURAL JOIN, JOIN…USING, or

JOIN…ON keywords in the FROM clause

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Equality Join: WHERE Clause Example

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Equality Join: NATURAL JOIN

Syntax: tablename NATURAL JOIN tablename

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Equality Join: JOIN…USING

Syntax: tablename JOIN tablename USING (columnname)

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Equality Join: JOIN…ON

Syntax: tablename JOIN tablename ON condition

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JOIN Keyword Overview

• Use NATURAL JOIN when tables have one column in common

• Use JOIN…USING when tables have more than one column in common

• Use JOIN…ON when a condition is needed to specify a relationship other than equivalency

• Using JOIN keyword frees the WHERE clause for exclusive use in restricting rows

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Non-Equality Joins

• In WHERE clause, use any comparison operator other than equal sign

• In FROM clause, use JOIN…ON keywords with non-equivalent condition

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Non-Equality Join: WHERE Clause Example

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Non-Equality Join: JOIN…ON Example

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Self-Joins

• Used to link a table to itself• Requires use of column qualifier

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Self-Join: WHERE Clause Example

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Self-Join: JOIN…ON Example

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Outer Joins //

• Use to include rows that do not have a match in the other table

• In WHERE clause, include outer join operator (+) next to table with missing rows to add NULL rows

• In FROM clause, use FULL, LEFT, or RIGHT with OUTER JOIN keywords

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Outer Join: WHERE Clause Example

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Outer Join: OUTER JOIN Keyword Example

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Set Operators XX

Used to combine the results of two or more SELECT statements

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Set Operator Example

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Joining Three or More Tables

• Same procedure as joining two tables• Will always results in one less join than the

number of tables being joined

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Joining Three or More Tables: Example