Base De Datos

Retrieving Data Using the SQL SELECT Statement

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Objectives

After completing this lesson, you should be able to do the following: • List the capabilities of SQL SELECT statements • Execute a basic SELECT statement • Differentiate between SQL statements and iSQL*Plus commands

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Capabilitiesof SQL SELECT Statements

Projection

Selection

Table 1 Join

Table 1

Table 1

Table 2
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Basic SELECT Statement

SELECT *|{[DISTINCT] column|expression [alias],...} FROM table;

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SELECT identifies the columns to be displayed FROM identifies the table containing those columns

Copyright © 2004, Oracle. All rights reserved.Selecting All Columns

SELECT * FROM departments;

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Selecting Specific Columns

SELECT department_id, location_id FROM departments;

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Writing SQL Statements

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SQL statements are not case-sensitive. SQL statements can be on one or more lines. Keywords cannot beabbreviated or split across lines. Clauses are usually placed on separate lines. Indents are used to enhance readability. In iSQL*Plus, SQL statements can optionally be terminated by a semicolon (;). Semicolons are required if you execute multiple SQL statements. In SQL*plus, you are required to end each SQL statement with a semicolon (;).

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ColumnHeading Defaults

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iSQL*Plus:
– Default heading alignment: Center – Default heading display: Uppercase

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SQL*Plus:
– Character and Date column headings are leftaligned – Number column headings are right-aligned – Default heading display: Uppercase

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Arithmetic Expressions

Create expressions with number and date data by usingarithmetic operators.
Operator + * / Description Add Subtract Multiply Divide

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Using Arithmetic Operators

SELECT last_name, salary, salary + 300 FROM employees;

…

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Operator Precedence

SELECT last_name, salary, 12*salary+100 FROM employees;

1

…
SELECT last_name, salary, 12*(salary+100)FROM employees;

2

…

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Defining a Null Value

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A null is a value that is unavailable, unassigned, unknown, or inapplicable. A null is not the same as a zero or a blank space.
SELECT last_name, job_id, salary, commission_pct FROM employees;

… …

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Null Values in ArithmeticExpressions
Arithmetic expressions containing a null value evaluate to null.
SELECT last_name, 12*salary*commission_pct FROM employees;

… …

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Defining a Column Alias

A column alias: • Renames a column heading • Is useful with calculations • Immediately follows the column name (There can also be the optional AS keyword between the column nameand alias.) • Requires double quotation marks if it contains spaces or special characters or if it is casesensitive

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Using Column Aliases

SELECT last_name AS name, commission_pct comm FROM employees;

…
SELECT last_name "Name" , salary*12 "Annual Salary" FROM employees;

…
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ConcatenationOperator

A concatenation operator: • Links columns or character strings to other columns • Is represented by two vertical bars (||) • Creates a resultant column that is a character expression
SELECT FROM last_name||job_id AS "Employees" employees;

…
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Literal Character Strings

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A literal is a character, a number, or a date that...