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Transcript of SQL

Introduction to SQLSQL is a standard language for accessing and manipulating databases.

What is SQL? SQL stands for Structured Query Language SQL lets you access and manipulate databases SQL is an ANSI (American National Standards Institute) standard

What Can SQL do? SQL SQL SQL SQL SQL SQL SQL SQL SQL SQL can can can can can can can can can can execute queries against a database retrieve data from a database insert records in a database update records in a database delete records from a database create new databases create new tables in a database create stored procedures in a database create views in a database set permissions on tables, procedures, and views

SQL is a Standard - BUT....Although SQL is an ANSI (American National Standards Institute) standard, there are many different versions of the SQL language. However, to be compliant with the ANSI standard, they all support at least the major commands (such as SELECT, UPDATE, DELETE, INSERT, WHERE) in a similar manner. Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!

Using SQL in Your Web SiteTo build a web site that shows some data from a database, you will need the following:

An RDBMS database program (i.e. MS Access, SQL Server, MySQL) A server-side scripting language, like PHP or ASP SQL

HTML / CSS

RDBMSRDBMS stands for Relational Database Management System. RDBMS is the basis for SQL, and for all modern database systems like MS SQL Server, IBM DB2, Oracle, MySQL, and Microsoft Access. The data in RDBMS is stored in database objects called tables. A table is a collection of related data entries and it consists of columns and rows.

SQL SyntaxDatabase TablesA database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with data. Below is an example of a table called "Persons": P_Id 1 2 3 LastName Hansen Svendson Pettersen FirstName Ola Tove Kari Address Timoteivn 10 Borgvn 23 Storgt 20 City Sandnes Sandnes Stavanger

The table above contains three records (one for each person) and five columns (P_Id, LastName, FirstName, Address, and City).

SQL StatementsMost of the actions you need to perform on a database are done with SQL statements. The following SQL statement will select all the records in the "Persons" table:SELECT * FROM Persons

In this tutorial we will teach you all about the different SQL statements.

Keep in Mind That...SQL is not case sensitive

Semicolon after SQL Statements?Some database systems require a semicolon at the end of each SQL statement. Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server. We are using MS Access and SQL Server 2000 and we do not have to put a semicolon after each SQL statement, but some database programs force you to use it.

SQL DML and DDLSQL can be divided into two parts: The Data Manipulation Language (DML) and the Data Definition Language (DDL). The query and update commands form the DML part of SQL:

SELECT - extracts data from a database UPDATE - updates data in a database DELETE - deletes data from a database INSERT INTO - inserts new data into a database

The DDL part of SQL permits database tables to be created or deleted. It also define indexes (keys), specify links between tables, and impose constraints between tables. The most important DDL statements in SQL are:

CREATE DATABASE - creates a new database ALTER DATABASE - modifies a database CREATE TABLE - creates a new table ALTER TABLE - modifies a table DROP TABLE - deletes a table CREATE INDEX - creates an index (search key) DROP INDEX - deletes an index

SQL SELECT StatementThis chapter will explain the SELECT and the SELECT * statements.

The SQL SELECT StatementThe SELECT statement is used to select data from a database. The result is stored in a result table, called the result-set.SQL SELECT SyntaxSELECT column_name(s) FROM table_name

and

SELECT * FROM table_name

Note: SQL is not case sensitive. SELECT is the same as select.

An SQL SELECT ExampleThe "Persons" table: P_Id 1 2 3 LastName Hansen Svendson Pettersen FirstName Ola Tove Kari Address Timoteivn 10 Borgvn 23 Storgt 20 City Sandnes Sandnes Stavanger

Now we want to select the content of the columns named "LastName" and "FirstName" from the table above. We use the following SELECT statement:SELECT LastName,FirstName FROM Persons

The result-set will look like this: LastName Hansen FirstName Ola

Svendson Pettersen

Tove Kari

SELECT * ExampleNow we want to select all the columns from the "Persons" table. We use the following SELECT statement:SELECT * FROM Persons

Tip: The asterisk (*) is a quick way of selecting all columns! The result-set will look like this:P_Id 1 2 3 LastName Hansen Svendson Pettersen FirstName Ola Tove Kari Address Timoteivn 10 Borgvn 23 Storgt 20 City Sandnes Sandnes Stavanger

Navigation in a Result-setMost database software systems allow navigation in the result-set with programming functions, like: Move-To-First-Record, Get-Record-Content, Move-To-Next-Record, etc. Programming functions like these are not a part of this tutorial. To learn about accessing data with function calls, please visit our ADO tutorial or our PHP tutorial.

SQL SELECT DISTINCT StatementThis chapter will explain the SELECT DISTINCT statement.

The SQL SELECT DISTINCT StatementIn a table, some of the columns may contain duplicate values. This is not a problem, however, sometimes you will want to list only the different (distinct) values in a table. The DISTINCT keyword can be used to return only distinct (different) values.

SQL SELECT DISTINCT SyntaxSELECT DISTINCT column_name(s) FROM table_name

SELECT DISTINCT ExampleThe "Persons" table: P_Id 1 2 3 LastName Hansen Svendson Pettersen FirstName Ola Tove Kari Address Timoteivn 10 Borgvn 23 Storgt 20 City Sandnes Sandnes Stavanger

Now we want to select only the distinct values from the column named "City" from the table above. We use the following SELECT statement:SELECT DISTINCT City FROM Persons

The result-set will look like this: City Sandnes Stavanger

SQL WHERE ClauseThe WHERE clause is used to filter records.

The WHERE ClauseThe WHERE clause is used to extract only those records that fulfill a specified criterion.

SQL WHERE SyntaxSELECT column_name(s) FROM table_name WHERE column_name operator value

WHERE Clause ExampleThe "Persons" table: P_Id 1 2 3 LastName Hansen Svendson Pettersen FirstName Ola Tove Kari Address Timoteivn 10 Borgvn 23 Storgt 20 City Sandnes Sandnes Stavanger

Now we want to select only the persons living in the city "Sandnes" from the table above. We use the following SELECT statement:SELECT * FROM Persons WHERE City='Sandnes'

The result-set will look like this: P_Id 1 2 LastName Hansen Svendson FirstName Ola Tove Address Timoteivn 10 Borgvn 23 City Sandnes Sandnes

Quotes Around Text FieldsSQL uses single quotes around text values (most database systems will also accept double quotes). Although, numeric values should not be enclosed in quotes. For text values:

This is correct: SELECT * FROM Persons WHERE FirstName='Tove' This is wrong: SELECT * FROM Persons WHERE FirstName=Tove

For numeric values:

This is correct: SELECT * FROM Persons WHERE Year=1965 This is wrong: SELECT * FROM Persons WHERE Year='1965'

Operators Allowed in the WHERE ClauseWith the WHERE clause, the following operators can be used: Operator = > < >= (SELECT AVG(OrderPrice) FROM Orders)

The result-set will look like this: Customer Hansen Nilsen Jensen

SQL COUNT() FunctionThe COUNT() function returns the number of rows that matches a specified criteria.

SQL COUNT(column_name) SyntaxThe COUNT(column_name) function returns the number of values (NULL values will not be counted) of the specified column:

SELECT COUNT(column_name) FROM table_name

SQL COUNT(*) SyntaxThe COUNT(*) function returns the number of records in a table:

SELECT COUNT(*) FROM table_name

SQL COUNT(DISTINCT column_name) SyntaxThe COUNT(DISTINCT column_name) function returns the number of distinct values of the specified column:

SELECT COUNT(DISTINCT column_name) FROM table_name

Note: COUNT(DISTINCT) works with ORACLE and Microsoft SQL Server, but not with Microsoft Access.

SQL COUNT(column_name) ExampleWe have the following "Orders" table: O_Id 1 2 3 4 5 6 OrderDate 2008/11/12 2008/10/23 2008/09/02 2008/09/03 2008/08/30 2008/10/04 OrderPrice 1000 1600 700 300 2000 100 Customer Hansen Nilsen Hansen Hansen Jensen Nilsen

Now we want to count the number of orders from "Customer Nilsen". We use the following SQL statement:

SELECT COUNT(Customer) AS CustomerNilsen FROM Orders WHERE Customer='Nilsen'

The result of the SQL statement above will be 2, because the customer Nilsen has made 2 orders in total: CustomerNilsen 2

SQL COUNT(*) ExampleIf we omit the WHERE clause, like this:

SELECT COUNT(*) AS NumberOfOrders FROM Orders

The result-set will look like this: NumberOfOrders 6

which is the total number of rows in the table.

SQL COUNT(DISTINCT column_name) ExampleNow we want to count the number of unique customers in the "Orders" table. We use the following SQL statement:

SELECT COUNT(DISTINCT Customer) AS NumberOfCustomers FROM Orders

The result-set will look like this: NumberOfCustomers 3

which is the number of unique customers (Hansen, Nilsen, and Jensen) in the "Orders" table.

SQL FIRS