what is the sql select statement

8/22/2019 What is the SQL SELECT Statement

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What is the SQL SELECT Statement?

The SQL SELECT statement is the SQL command that retrieves data from an SQL database. Thisoperation is also known as a query and is the key to the use of SQL for analysis and reporting. Thistutorial section shows the basic SQL SELECT statement so see theSQL Advancedtutorial section formore in depth knowledge.

Why Use the SQL SELECT Statement?

Anytime you want to retrieve information from a SQL database use the SQL SELECT statement torequest the information.

How To Use the SQL SELECT Statement

The SQL SELECT command is used as follows. The asterisk can be used to SELECT all columns of atable.

SQL SELECT Statement Syntax

SELECT FROM

SQL SELECT Statement Example 1

The following example retrieves the columns product_nbr and product_name from all rows ofthe product table.

SELECT product_nbr, product_nameFROM product

Results from the execution of the SQL SELECT statement, also known as a query are as follows:

product_nbrproduct_name

1001 SQL Tool 1.0

2001 SQL Tool 2.0 Light

2002 SQL Tool 2.0 Professional

2003 SQL Tool 2.0 Enterprise

SQL SELECT Statement Example 2

The following example retrieves all of the columns from all rows of the product table.

SELECT *FROM product
http://infogoal.com/sql/sql-advanced.htmhttp://infogoal.com/sql/sql-advanced.htmhttp://infogoal.com/sql/sql-advanced.htmhttp://infogoal.com/sql/sql-advanced.htm


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Results show the product_status_code column that was not present in Example 1.

product_nbrproduct_name product_status_code

1001 SQL Tool 1.0 Inactive

2001 SQL Tool 2.0 Light Active

2002 SQL Tool 2.0 Professional Active

2003 SQL Tool 2.0 Enterprise Active

How To Use the SQL CREATE DATABASE Statement

The SQL CREATE DATABASE statement is used as follows.

SQL CREATE DATABASE Statement Syntax

CREATE DATABASE

SQL CREATE DATABASE Statement Example

The following example creates a new database named CUSTOMER_DB.

CREATE DATABASE customer_db

What is the SQL CREATE INDEX Statement?

The SQL INDEX TABLE statement is the SQL command that adds a new index to an existing table to anSQL database. Indexed enable faster and more efficient retrieval of information from SQL databases.

Why Use the SQL CREATE INDEX Statement?

SQL indexes are used because they can provide the following benefits / functions:

Rapid access of information

Efficient access of information

Enforcement of uniqueness constraints

Correct use of indexes can make the difference between a top performing database with highcustomer satisfaction and a non-performing database with low customer satisfaction.

How To Use the SQL CREATE INDEX Statement

The SQL CREATE INDEX command is used as follows.


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SQL CREATE INDEX Statement Syntax

CREATE INDEX ON (

,

,

)

CREATE UNIQUE INDEX ON (

,

,

)

The number of characters that can make up SQL names for tables, columns and indexes varies byDBMS. In many cases the limit is 30 characters. The leading character of the name must be alphabetic -not a number or special character. The name of a new index can not duplicate the name of an existing

index for the same table and should not be the same as a SQL reserved word. The underscore charactercan be used to improve readability. List elements are seperated by commas.

SQL CREATE INDEX Statement Example 1 - Rapid Access

The following example creates an non-unique index named IDX_PERSON_NAME ontable PERSON with columns named person_name and gender_code in ascending sequence..

Here are the contents of the table:

Column Name Datatype Nullability

person_id INT NOT NULL

person_name VARCHAR(20) NOT NULL

social_security_nbr CHAR(9) NULL

gender_code CHAR(1) NULL

line_1_addr VARCHAR(50) NULL

city_name VARCHAR(50) NOT NULL

state_code CHAR(2) NOT NULL

zip_code CHAR(9) NOT NULL

This SQL CREATE VIEW Statement is executed:

CREATE INDEX IDX_PERSON_NAME ON PERSON


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SQL CREATE INDEX Statement Example 2 - Enforce Uniqueness

The following example creates a unique index named IDX_PERSON_SSN on table PERSON with thecolumn named social_security_nnbr. This assures that two rows can not be added to the PERSONtable with the same social security number.

This SQL CREATE TABLE Statement is executed:


What is the SQL CREATE TABLE Statement?

The SQL CREATE TABLE statement is the SQL command that adds a new table to an SQL database.Tables are a basic unit of organization and storage of data in SQL. Each table tends to represent anentity such as a customer, product, code or event. A table is similar to a file in a non-database system.

Tables are organized into rows and columns. For example, a customer table would likely have a seperaterow for each customer. Columns are attributes that describe a row. For example, a row in a customertable would have columns like customer_name, customer_nbr, account_balance_amt andestablished_date.

Why Use the SQL CREATE TABLE Statement?

Anytime you want to add a new table to the database you would use the SQL CREATE TABLEstatement.

How To Use the SQL CREATE TABLE Statement

The SQL CREATE TABLE command is used as follows.

SQL CREATE TABLE Statement Syntax

CREATE TABLE ( )

The number of characters that can make up SQL table names and column names varies by DBMS. Inmany cases the limit is 30 characters. The leading character of the name must be alphabetic - not a


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number or special character. The name of a new table can not duplicate the name of an existing tableand should not be the same as a SQL reserved word. The underscore character can be used to improvereadability. The same column name can not be repeated within a table. List elements are seperated bycommas.

Here are some example datatypes:

SQL Datatype Description

integer(size)int(size)smallint(size)tinyint(size)

Integers

decimal(size,decimals)numeric(size,decimals)

Numbers with decimals

char(size) Fixed length character string

varchar(size) Variable length character string

date A date in yyyymmdd format

SQL CREATE TABLE Statement Example

The following example creates a table named product with columnsnamed product_nbr, product_name, product_status_code, start_date, end_date andraw_material_cost_amt.

This SQL CREATE TABLE Statement is executed:

CREATE TABLE product ( product_nbr int NOT NULL,product_name varchar(40) NOT NULL,product_status_code char(10) NOT NULL,start_date date NULL,

end_date date NULL,raw_material_cost decimal(12,2) NULL,primary key (product_nbr))

What is the SQL CREATE VIEW Statement?

A SQL view is a virtual table and the SQL CRERATE VIEW statement is the SQL command that adds anew view to a SQL database.

A view can be accessed using the SQL SELECT statement like a table. A view is built by selecting data

from one or more tables.

Some views can also support the SQL INSERT, SQL UPDATE and SQL DELETE statements. In thatcase, the view must refer to a single table and include all NOT NULL columns of that table.

Why Use the SQL CREATE VIEW Statement?

SQL views are used because they can provide the following benefits / functions:


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Database queries are simplified

Database complexity is hidden

Flexibility is increased - queries of views may not change when underlying tables chagne

Security is increased - sensitive information can be excluded from a view

How To Use the SQL CREATE VIEW Statement

The SQL CREATE VIEW command is used as follows.

SQL CREATE VIEW Statement Syntax

CREATE VIEW (,) AS

The number of characters that can make up SQL names for tables, columns and views varies by DBMS.In many cases the limit is 30 characters. The leading character of the name must be alphabetic - not anumber or special character. The name of a new view can not duplicate the name of an existing view ortable and should not be the same as a SQL reserved word. The underscore character can be used toimprove readability. List elements are seperated by commas.

SQL CREATE VIEW Statement Example - Data Filtering

The following example creates a view named V_TOP_CUSTOMER on table CUSTOMER with thefollowing columns:

customer_id

customer_name ytd_sales_amt

zip_code

Here are the contents of the table:


customer_id INT NOT NULL

customer_name VARCHAR(20) NOT NULL

ytd_sales_amt MONEY NOT NULL





This SQL CREATE VIEW Statement is executed:

CREATE VIEW V_TOP_CUSTOMER (customer_id,


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customer_name,ytd_sales_amt,zip_code)AS SELECT customer_id,customer_name,ytd_sales_amt,zip_codeFROM CUSTOMERWHERE ytd_sales_amt > 1200.00

What is the SQL ALTER TABLE Statement?

The SQL ALTER TABLE statement is the SQL command that makes changes to the definition of an SQLtable.

Why Use the SQL ALTER TABLE Statement?

Anytime you want to change the definition of an SQL table. For example, you could:

Add a column to a table

Change the definition of an existing column in a table

Drop a column from a table

How To Use the SQL ALTER TABLE Statement

The SQL ALTER TABLE command is used as follows.

SQL ALTER TABLE Statement Syntax

ALTER TABLE ADD

ALTER TABLE ALTER COLUMN

ALTER TABLE DROP COLUMN

SQL ALTER TABLE Statement Example 1 - Add a Column

The following example adds a new column into the person table. Before the operation takes place thefollowing columns exists in the table:

Column NameDatatype Nullability




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apartment_nbr VARCHAR(50) NULL




This ALTER TABLE Statement is executed:

ALTER TABLE PERSONADD marital_status_code CHAR(1) NULL

Results from the execution of the SQL ALTER TABLE statement are:



person_name ARCHAR(20) NOT NULL


line_1_addr ARCHAR(50) NULL

line_2_addr ARCHAR(50) NULL

apartment_nbr ARCHAR(50) NULL

city_name ARCHAR(50) NOT NULL



marital_status_codeCHAR(1) NULL

SQL ALTER TABLE Statement Example 2 - Alter a Column Datatype

The following example changes the datatype of an existing column in the person table. Before theoperation takes place the following columns exists in the table:








city_name VARCHAR(50) NOT NULLstate_code CHAR(2) NOT NULL



ALTER TABLE PERSONALTER COLUMN person_name VARCHAR(50) NOT NULL


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person_name VARCHAR(50)NOT NULL








marital_status_code CHAR(1) NULL

SQL ALTER TABLE Statement Example 3 - Drop a column

The following example removes an existing column in the person table. Before the operation takes placethe following columns exists in the table:









state_code CHAR(2) NOT NULLzip_code CHAR(9) NOT NULL


ALTER TABLE dbo.PERSONDROP COLUMN apartment_nbr


Column NameDatatype Nullabilityperson_id INT NOT NULL









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What is SQL JOIN?

The SQL JOIN is a clause that enables a SELECT statement to access more than one table. The JOIN

clause controls how tables are linked. It is a qualifier of the SQL FROM clause.

The standard JOIN clause (also known as the INNER JOIN clause) differs from the OUTER JOIN in thatrows are returned only when there are matches for the JOIN critieria on the second table.

Why Use SQL JOIN?

Use the SQL JOIN whenever multiple tables must be accessed through a SQL SELECT statement andno results should be returned if there is not a match between the JOINed tables.

How To Use SQL JOIN

SQL JOIN is used as follows. The ON clause describes the conditions of the JOIN.

Important! A "cartesian product" can result if there is no relating the tables for the join. A row would beincluded for each combination between the two tables so if one table has 1,000 rows and the secondtable has 2,000 rows then 2,000,000 rows would be returned.

Important! If there are no matches on the JOIN criteria then no rows will be returned. This is known an"INNER JOIN". Use the "OUTER JOIN" in cases where rows should be returned when one side of the joinis missing.

SQL JOIN Syntax

SELECT , FROM JOIN ON

SQL JOIN Example

The following example JOINs the region and branch tables on the region_nbr column.

Here are the contents of the tables:

Table: REGION

region_nbrregion_name

100 East Region

200 Central Region

300 Virtual Region

400 West Region

Table: BRANCH


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branch_nbrbranch_nameregion_nbremployee_count

108 New York 100 10

110 Boston 100 6

212 Chicago 200 5

404 San Diego 400 6

415 San Jose 400 3

This SQL Statement with JOIN is executed:

SELECT region.region_nbr, region.region_name, branch.branch_nbr, branch.branch_nameFROM dbo.regionJOIN dbo.branchON branch.region_nbr = region.region_nbrORDER BY region.region_nbr

Here is the result. Note that the "Virtual Region" is included in the results even though it has no rows in

the branch table. This is the difference between the INNER JOIN and OUTER JOIN.

region_nbrregion_name branch_nbrbranch_name

100 East Region 108 New York

100 East Region 110 Boston

200 Central Region 212 Chicago

400 West Region 404 San Diego

400 West Region 415 San Jose

What is SQL OUTER JOIN?

The SQL OUTER JOIN clause is a variation of the SQL JOIN clause enables a SELECT statement toaccess more than one table. The JOIN clause controls how tables are linked. It is a qualifier of the SQLFROM clause.

The OUTER JOIN clause differs from the standard JOIN clause (also known as the INNER JOIN clause)in that rows are returned even when there are no matches through the JOIN critieria on the second table.

Why Use SQL OUTER JOIN?

Use the SQL OUTER JOIN whenever multiple tables must be accessed through a SQL SELECTstatement and results should be returned if there is not a match between the JOINed tables.

It can be useful when there is a need to merge data from two tables and to include all rows from both

tables without depending on a match. Another use is to generate a large result set for testing purposes.

How To Use SQL OUTER JOIN

SQL OUTER JOIN is used as follows. The ON clause describes the conditions of the JOIN.


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Important! A "cartesian product" can result if there is no relating the tables for the join. A row would beincluded for each combination between the two tables so if one table has 1,000 rows and the secondtable has 2,000 rows then 2,000,000 rows would be returned.

Important! If there are matches on the JOIN criteria then rows will still be returned. This is known an"OUTER JOIN". Use the "INNER JOIN" in cases where no rows should be returned when one side of the

join is missing.

SQL OUTER JOIN Syntax

SELECT , FROM LEFT OUTER JOIN ON

SQL OUTER JOIN Example

The following example JOINs the region and branch tables on the region_nbr column.

Here are the contents of the tables:

Table: REGION

region_nbrregion_name

100 East Region

200 Central Region

300 Virtual Region

400 West Region

Table: BRANCH

branch_nbrbranch_nameregion_nbremployee_count

108 New York 100 10

110 Boston 100 6

212 Chicago 200 5

404 San Diego 400 6

415 San Jose 400 3

This SQL Statement with OUTER JOIN is executed:

SELECT region.region_nbr, region.region_name, branch.branch_nbr, branch.branch_nameFROM dbo.regionLEFT OUTER JOIN dbo.branchON branch.region_nbr = region.region_nbrORDER BY region.region_nbr

Here is the result. Note that the "Virtual Region" is included in the results even though it has no rows inthe branch table. This is the difference between the INNER JOIN and OUTER JOIN.


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region_nbrregion_name branch_nbrbranch_name

100 East Region 108 New York

100 East Region 110 Boston

200 Central Region 212 Chicago

300 Virtual Region NULL NULL

400 West Region 404 San Diego

400 West Region 415 San Jose

what is the sql select statement

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