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SQL query
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SQL query Structured query
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Chapter 1Writing Basic
SQL Statements
1
, 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 SQL*Plus
commands
Lesson Aim
To extract data from the database. you need to use the structured query
language (SQL) SELECT statement You may need to restrict the columns
that are displayed. This lesson describes all Ihe SQL statements that you need
to perform these actions.
You may want to create SELECT statements that can be used time and time
again. Tins lesson also covers the use of SQL*Plus commands to execute
SQL statements
2
, Capabilities of SQL SELECT
Statements
Selection Projection
Table
2
Table
1
Capabilities of SQL SELECT Statements
A SELECT statement retrieves information from the database. Using a
SELECT statement, you can do the following:
Selection: You can use the selection capability in SQL to choose the rows in
a table that you want returned by a query. You can use various criteria to
selectively restrict the rows that you see.
Projection: You can use the projection capability in SQL to choose the
columns in a table that you want returned by your query. You can choose as
few or as many columns of the table as you require.
Join: You can use the join capability in SQL to bring together data that is
stored in different tables by creating a link through a column that both the
tables share. You will learn more about joins in a later lesson.
3
, Basic SELECT Statement
SELECT [DISTINCT] {*, column [alias],..,}
FROM table;
•SELECT identifies what columns.
•FROM identifies which table.
Basic SELECT Statement
In its simplest form, a SELECT statement must include the following
A SELECT clause, which specifies the columns to be displayed.
A FROM clause, which specifies the table containing the columns
listed in the SELECT clause.
In the syntax:
SELECT is a list of one or more columns.
DISTINCT suppresses duplicates.
* selects all columns
column selects the named column.
alias gives selected columns different headings.
FROM table specifies the table containing the columns.
Note: Throughout this course, the words keyword, clause, and statement are
used.
A keyword refers to an individual SQL element. For example, SELECT and
FROM are keywords.
A clause is a part of an SQL statement. For example. SELECT empno,
ename, … is a clause.
A statement is a combination of two or more clauses. For example. SELECT
* FROM emp is a SQL statement.
4
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