Download The SELECT Command

Introduction to Databases
Chapter 6:
Understanding the SQL
Language
Introducing the SQL Language
• Two primary aspects:
– Data definition:
• Requires data definition language (DDL)
that defines tables, attributes, and indexes
in DBMS
– Data manipulation:
• Requires data manipulation language
(DML) for issuing data manipulation
commands to DBMS
Introducing the SQL Language
• Data retrieval statements:
– Sometimes placed in different category,
(separate from DML), as data query language
(DQL) statements
• SQL:
– Standard current relational DBMS language
– Incorporates DDL and DML features
SQL Features
• ANSI SQL standard
• Basic features of standard SQL:
– Data definition language (DDL): Used to create and
maintain database objects
– Data manipulation language (DML): Used to retrieve and
manipulate data
– Command operators: Symbols and keywords used to
run arithmetic, comparison, and logical operations
– Functions: Special executables that return values
– Transaction control: Statements to initiate and complete
or abort transaction processing
Using SQL
• Two basic options for executing SQL
commands:
– Interactive SQL:
• SQL statements that you run directly,
interacting with database server
– Embedded SQL:
• SQL functionality embedded in procedure or
part of application written in different
programming language
Interactive SQL
• All DBMS provide command interface or
command prompt for running interactive
SQL commands
• Microsoft SQL Server - Two options:
– Command prompt: Issuing statements one at a time or
several in set or batch
– Loading and running scripts, groups of SQL Server
commands stored as file
• Text-based interfaces: Complex, but allow
execution of SQL language commands
through batch files
• Some DBMS also have graphic user
interface (GUI)
Sqlcmd Command Interface
SQL Server Management Studio
Embedded SQL
• Uses same SQL language commands as
interactive SQL statements
• Are included as part of executable
program
– For example: SQL Server’s stored procedures
and user-defined functions, which provide
embedded SQL support
• Also used in application programs:
– Programming environment provides
connectivity tools to communicate with
database server and execute SQL commands
Command Basics
• Command syntax:
– Includes command name and any command
keywords and parameters
• Command keywords:
– Describe specific actions that command will
take
• Command parameters:
– Values supplied so command can run
• Example:
SELECT select_list FROM table [WHERE
qualifying_conditions]
Command Basics
• Commands typically run in context of
default database
– Each connection will have associated user
• Query results vary according to command:
– SELECT command returns result set (relational
result) of columns and rows
– DML statements usually return count of rows
affected
The SELECT Command
• SELECT commands: Declarative
statements (not procedural)
• Relational DBMS:
– Analyzes SQL SELECT statement
– Creates access path—plan for steps to take in
responding to query
• SQL SELECT commands can be run in
interactive query or embedded mode
Example: Simple Data Retrieval
Example: Simple Data Retrieval
To find commission percentage and year of hire of salesperson
186:
SELECT COMMPERCT, YEARHIRE FROM
SALESPERSON WHERE SPNUM=186
To retrieve entire record for salesperson 186:
SELECT * FROM SALESPERSON WHERE SPNUM=186
To list salesperson number and salesperson name of all of
salespersons:
SELECT SPNUM, SPNAME FROM SALESPERSON
To return all rows and columns of data in Salesperson table:
SELECT * FROM SALESPERSON
Retrieving Other Values
• In Microsoft SQL Server, SELECT statement can
be used to evaluate expressions
– Syntax:
SELECT expression
• Expression: Any mathematical or logical
expression that returns result
• For example:
SELECT 5 + 7
will return “12”
• Syntax for resolving function:
SELECT function [(parameter_list)]
Operators and Functions
• SQL operators:
– Arithmetic operators
– Logical operators
• Used in WHERE clause to build search conditions that depend on
more than one qualifying condition
• Unary operator:
– Operator applied to only one operand
– Typical format : operator operand
– For example: NOT A
• Binary operator:
– Operator applied to two operands
– Typical format: operand operator operand
– For example: A OR B
Arithmetic Operators
Arithmetic Operators
• To add two values:
SELECT 5+ 5 total_value
• To concatenate two values as strings:
SELECT ‘5’ + ‘5’ total_value
• Data type may affect precision and scale
of result of arithmetic operation
Comparison Operators
Comparison Operators
• Can be used:
– In SELECT statement’s WHERE clause
– With decision statements; for example:
• If A>B, then perform action
– By themselves or together with logical
operators
Logical Operators
Logical Operators
• Used to evaluate set of conditions
• Return results of TRUE, FALSE, or
“unknown”
• Can be used:
– In SELECT statement WHERE clause
– With decision statements; for example:
• If A AND B are true, then perform action
Example: Comparison and Logical Operators
Example: Comparison and Logical Operators
• To list customer numbers, customer names, and
headquarter cities of customers that are
headquartered in New York or that satisfy two
conditions: having customer number higher than
1500 and being headquartered in Atlanta
SELECT CUSTNUM, CUSTNAME, HQCITY FROM
CUSTOMER WHERE HQCITY=’New York’ OR
(CUSTNUM>1500 AND HQCITY=’Atlanta’)
• Note that:
– AND has higher precedence than OR
– Clauses in parentheses evaluated first
The Like Operator
The Like Operator
• Used when criteria in WHERE clause of
SELECT query are only partially known
– For example, to find records for customers
whose names begin with letter A
SELECT * FROM CUSTOMER WHERE
CUSTNAME LIKE ‘A%’
Standard SQL Functions
• Functions supported by various DBMS providers
vary widely from ANSI standard function list and
from each other
• Deterministic functions:
– Always return same result if you pass in same
arguments
– E.g. ABS: Returns absolute value of number
• Nondeterministic functions:
– May return different results when called with same
arguments
– E.g. GETDATE( ): Accepts no arguments and returns
only current date and time
General Function Categories
• Numeric
– Functions that operate with numbers
– Statistical functions (e.g. mean and standard
deviations)
– Random number generator
• Mathematical
– Perform specific mathematic calculations
– For example, trigonometric functions for SIN,
COS, and so forth
General Function Categories
• Date/Time
– Performing calculations on dates and times, for
example:
• Current date/time information
• Formatting date and time strings
• Performing calculations based on date and time
• Aggregate
– Used to generate aggregate values based on
sets of numeric values
– For example, generating total, count, or
average
General Function Categories
• String
– Perform string manipulations such as:
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Modifying strings
Locating patterns within string
Inserting characters
Concatenating strings
Finding string length
Changing character case
• Conversion
– Used to convert values between data types
– Allow explicit conversions between otherwise
incompatible types
Sample SQL Functions
• Standard SQL-99 functions include:
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BIT_LENGTH
CAST
CURRENT_DATE
SUBSTRING
UPPER
• SQL Server built-in aggregate functions include:
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AVG
COUNT
MAX
SUM
DML Commands
• Three basic DML statements (ANSI SQL):
– INSERT:
• To add rows to table
– UPDATE:
• To modify values in existing columns and rows
– DELETE:
• To remove rows from tables
• Statements may be subject to constraints
or other limits placed on table
– E.g. Foreign key constraint may disallow
deletion of row referenced in another table
DDL Commands
• DDL commands are used to create and
manage server and database objects
– Server objects: Objects implemented at server
level
– Database objects: Database-specific objects,
such as tables
• DDL commands vary widely in how they
are implemented by various DBMSs
DDL Commands
• Three basic object management
commands:
– CREATE
• To create server and database objects, such as:
– CREATE TABLE and CREATE INDEX
– ALTER:
• To modify server and database objects, such as:
– ALTER TABLE and ALTER INDEX
– DROP:
• To delete server and database objects, such as:
– DROP TABLE and DROP INDEX
Summary
• SQL: Standard relational DBMS language developed
by ANSI that incorporates data definition language
(DDL) and DML (data manipulation language (DML)
features
• SQL commands follow specified syntax structure,
using command name, command keywords, and
parameters
• SQL commands feature arithmetical, comparison, and
logical operators to modify queries and results
• Different SQL vendors may provide different functions,
in categories such as mathematical, numeric,
date/time, aggregate, string, conversion
• DML commands include INSERT, UPDATE, and
DELETE
• DDL commands include CREATE, ALTER, and DROP
Key Terms
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Access path
Aggregate function
Batch
Binary Operator
Built-in functions
Clause
Command operators
Command syntax
Data definition language
(DDL)
• Data manipulation
language (DML)
• Data query language
(DQL)
• Declarative statement
• Default database
• Deterministic function
• Domain
• Dynamic SQL
• Embedded SQL
• Explicit conversion
• Function
• Implicit conversion
• Interactive SQL
Key Terms
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Keyword
Minimally logged operation
Non-logged operation
Nondeterministic function
Operator precedence
Parameters
Parsing
Procedure
Qualifying conditions
Query mode
Relational result
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Result set
Script
Search argument
Sqlcmd
Subquery
Temporary table
Transaction control
User-defined data type
Unary operator
Working database
Copyright Notice
Copyright 2008 John Wiley & Sons, Inc.
All rights reserved. Reproduction or translation of this
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