SQL

SQL SELECT DISTINCT STATEMENT

Deepanshu Mehta — Sun, 11 Oct 2009 17:43:56 +0000

In 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 syntax

SELECT DISTINCT column_name(s)
FROM table_name

SELECT DISTINCT Example

The “Persons” table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	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 Clause

Deepanshu Mehta — Sun, 11 Oct 2009 17:42:20 +0000

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

SQL WHERE Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name operator value

WHERE Clause Example

The “Persons” table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	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	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes

Quotes Around Text Fields

SQL 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 Clause

With the WHERE clause, the following operators can be used:

Operator	Description
=	Equal
<>	Not equal
>	Greater than
<	Less than
>=	Greater than or equal
<=	Less than or equal
BETWEEN	Between an inclusive range
LIKE	Search for a pattern
IN	If you know the exact value you want to return for at least one of the columns

Note: In some versions of SQL the <> operator may be written as !=

SQL AND & OR OPERATORS

Deepanshu Mehta — Sun, 11 Oct 2009 17:40:01 +0000

The AND & OR Operators

The AND operator displays a record if both the first condition and the second condition is true.

The OR operator displays a record if either the first condition or the second condition is true.

AND Operator Example

The “Persons” table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger

Now we want to select only the persons with the first name equal to “Tove” AND the last name equal to “Svendson”:

We use the following SELECT statement:

SELECT * FROM Persons
WHERE FirstName=’Tove’
AND LastName=’Svendson’

The result-set will look like this:

P_Id	LastName	FirstName	Address	City
2	Svendson	Tove	Borgvn 23	Sandnes

OR Operator Example

Now we want to select only the persons with the first name equal to “Tove” OR the first name equal to “Ola”:

We use the following SELECT statement:

SELECT * FROM Persons
WHERE FirstName=’Tove’
OR FirstName=’Ola’

The result-set will look like this:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes

Combining AND & OR

You can also combine AND and OR (use parenthesis to form complex expressions).

Now we want to select only the persons with the last name equal to “Svendson” AND the first name equal to “Tove” OR to “Ola”:

We use the following SELECT statement:

SELECT * FROM Persons WHERE
LastName=’Svendson’
AND (FirstName=’Tove’ OR FirstName=’Ola’)

The result-set will look like this:

P_Id	LastName	FirstName	Address	City
2	Svendson	Tove	Borgvn 23	Sandnes

SQL ORDER BY KEYWORD

Deepanshu Mehta — Sun, 11 Oct 2009 17:39:11 +0000

The ORDER BY keyword is used to sort the result-set by a specified column.

The ORDER BY keyword sort the records in ascending order by default.

If you want to sort the records in a descending order, you can use the DESC keyword.

SQL ORDER BY Syntax

SELECT column_name(s)
FROM table_name
ORDER BY column_name(s) ASC|DESC

ORDER BY Example

The “Persons” table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger
4	Nilsen	Tom	Vingvn 23	Stavanger

Now we want to select all the persons from the table above, however, we want to sort the persons by their last name.

We use the following SELECT statement:

SELECT * FROM Persons
ORDER BY LastName

The result-set will look like this:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
4	Nilsen	Tom	Vingvn 23	Stavanger
3	Pettersen	Kari	Storgt 20	Stavanger
2	Svendson	Tove	Borgvn 23	Sandnes

ORDER BY DESC Example

Now we want to select all the persons from the table above, however, we want to sort the persons descending by their last name.

We use the following SELECT statement:

SELECT * FROM Persons
ORDER BY LastName DESC

The result-set will look like this:

P_Id	LastName	FirstName	Address	City
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger
4	Nilsen	Tom	Vingvn 23	Stavanger
1	Hansen	Ola	Timoteivn 10	Sandnes

SQL INSERT INTO statement

Deepanshu Mehta — Sun, 11 Oct 2009 17:38:28 +0000

The INSERT INTO statement is used to insert a new row in a table.

SQL INSERT INTO Syntax

It is possible to write the INSERT INTO statement in two forms.

The first form doesn’t specify the column names where the data will be inserted, only their values:

INSERT INTO table_name
VALUES (value1, value2, value3,…)

The second form specifies both the column names and the values to be inserted:

INSERT INTO table_name (column1, column2, column3,…)
VALUES (value1, value2, value3,…)

SQL INSERT INTO Example

We have the following “Persons” table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger

Now we want to insert a new row in the “Persons” table.

We use the following SQL statement:

INSERT INTO Persons
VALUES (4,’Nilsen’, ‘Johan’, ‘Bakken 2′, ‘Stavanger’)

The “Persons” table will now look like this:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger
4	Nilsen	Johan	Bakken 2	Stavanger

Insert Data Only in Specified Columns

It is also possible to only add data in specific columns.

The following SQL statement will add a new row, but only add data in the “P_Id”, “LastName” and the “FirstName” columns:

INSERT INTO Persons (P_Id, LastName, FirstName)
VALUES (5, ‘Tjessem’, ‘Jakob’)

The “Persons” table will now look like this:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger
4	Nilsen	Johan	Bakken 2	Stavanger
5	Tjessem	Jakob

SQL UPDATE STATEMENT

Deepanshu Mehta — Sun, 11 Oct 2009 17:37:28 +0000

The UPDATE statement is used to update existing records in a table.

SQL UPDATE Syntax

UPDATE table_name
SET column1=value, column2=value2,…
WHERE some_column=some_value

Note: Notice the WHERE clause in the UPDATE syntax. The WHERE clause specifies which record or records that should be updated. If you omit the WHERE clause, all records will be updated!

SQL UPDATE Example

The “Persons” table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger
4	Nilsen	Johan	Bakken 2	Stavanger
5	Tjessem	Jakob

Now we want to update the person “Tjessem, Jakob” in the “Persons” table.

We use the following SQL statement:

UPDATE Persons
SET Address=’Nissestien 67′, City=’Sandnes’
WHERE LastName=’Tjessem’ AND FirstName=’Jakob’

The “Persons” table will now look like this:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger
4	Nilsen	Johan	Bakken 2	Stavanger
5	Tjessem	Jakob	Nissestien 67	Sandnes

SQL UPDATE Warning

Be careful when updating records. If we had omitted the WHERE clause in the example above, like this:

UPDATE Persons
SET Address=’Nissestien 67′, City=’Sandnes’

The “Persons” table would have looked like this:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Nissestien 67	Sandnes
2	Svendson	Tove	Nissestien 67	Sandnes
3	Pettersen	Kari	Nissestien 67	Sandnes
4	Nilsen	Johan	Nissestien 67	Sandnes
5	Tjessem	Jakob	Nissestien 67	Sandnes

THE DELETE STATEMENT

Deepanshu Mehta — Sun, 11 Oct 2009 17:36:37 +0000

The DELETE Statement

The DELETE statement is used to delete rows in a table.

SQL DELETE Syntax

DELETE FROM table_name
WHERE some_column=some_value

Note: Notice the WHERE clause in the DELETE syntax. The WHERE clause specifies which record or records that should be deleted. If you omit the WHERE clause, all records will be deleted!

SQL DELETE Example

The “Persons” table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger
4	Nilsen	Johan	Bakken 2	Stavanger
5	Tjessem	Jakob	Nissestien 67	Sandnes

Now we want to delete the person “Tjessem, Jakob” in the “Persons” table.

We use the following SQL statement:

DELETE FROM Persons
WHERE LastName=’Tjessem’ AND FirstName=’Jakob’

The “Persons” table will now look like this:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger
4	Nilsen	Johan	Bakken 2	Stavanger

Delete All Rows

It is possible to delete all rows in a table without deleting the table. This means that the table structure, attributes, and indexes will be intact:

DELETE FROM table_name

DELETE * FROM table_name

Note: Be very careful when deleting records. You cannot undo this statement!

SQL EXERCISES

Deepanshu Mehta — Sun, 11 Oct 2009 17:34:19 +0000

Test your SQL Skills

On this page you can test your SQL skills.

We will use the Customers table in the Northwind database:

CompanyName	ContactName	Address	City
Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin
Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå
Centro comercial Moctezuma	Francisco Chang	Sierras de Granada 9993	México D.F.
Ernst Handel	Roland Mendel	Kirchgasse 6	Graz
FISSA Fabrica Inter. Salchichas S.A.	Diego Roel	C/ Moralzarzal, 86	Madrid
Galería del gastrónomo	Eduardo Saavedra	Rambla de Cataluña, 23	Barcelona
Island Trading	Helen Bennett	Garden House Crowther Way	Cowes
Königlich Essen	Philip Cramer	Maubelstr. 90	Brandenburg
Laughing Bacchus Wine Cellars	Yoshi Tannamuri	1900 Oak St.	Vancouver
Magazzini Alimentari Riuniti	Giovanni Rovelli	Via Ludovico il Moro 22	Bergamo
North/South	Simon Crowther	South House 300 Queensbridge	London
Paris spécialités	Marie Bertrand	265, boulevard Charonne	Paris
Rattlesnake Canyon Grocery	Paula Wilson	2817 Milton Dr.	Albuquerque
Simons bistro	Jytte Petersen	Vinbæltet 34	København
The Big Cheese	Liz Nixon	89 Jefferson Way Suite 2	Portland
Vaffeljernet	Palle Ibsen	Smagsløget 45	Århus
Wolski Zajazd	Zbyszek Piestrzeniewicz	ul. Filtrowa 68	Warszawa

To preserve space, the table above is a subset of the Customers table used in the example below.

Try it Yourself

To see how SQL works, you can copy the SQL statements below and paste them into the textarea, or you can make your own SQL statements.

SELECT * FROM customers

SELECT CompanyName, ContactName FROM customers

SELECT * FROM customers WHERE companyname LIKE ‘a%’

SELECT CompanyName, ContactName
FROM customers
WHERE CompanyName > ‘a’

When using SQL on text data, “alfred” is greater than “a” (like in a dictionary).

SELECT CompanyName, ContactName
FROM customers
WHERE CompanyName > ‘g’
AND ContactName > ‘g’

SELECT * FROM customers

SQL TRY IT

Deepanshu Mehta — Sun, 11 Oct 2009 17:33:27 +0000

Test your SQL Skills

On this page you can test your SQL skills.

We will use the Customers table in the Northwind database:

CompanyName	ContactName	Address	City
Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin
Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå
Centro comercial Moctezuma	Francisco Chang	Sierras de Granada 9993	México D.F.
Ernst Handel	Roland Mendel	Kirchgasse 6	Graz
FISSA Fabrica Inter. Salchichas S.A.	Diego Roel	C/ Moralzarzal, 86	Madrid
Galería del gastrónomo	Eduardo Saavedra	Rambla de Cataluña, 23	Barcelona
Island Trading	Helen Bennett	Garden House Crowther Way	Cowes
Königlich Essen	Philip Cramer	Maubelstr. 90	Brandenburg
Laughing Bacchus Wine Cellars	Yoshi Tannamuri	1900 Oak St.	Vancouver
Magazzini Alimentari Riuniti	Giovanni Rovelli	Via Ludovico il Moro 22	Bergamo
North/South	Simon Crowther	South House 300 Queensbridge	London
Paris spécialités	Marie Bertrand	265, boulevard Charonne	Paris
Rattlesnake Canyon Grocery	Paula Wilson	2817 Milton Dr.	Albuquerque
Simons bistro	Jytte Petersen	Vinbæltet 34	København
The Big Cheese	Liz Nixon	89 Jefferson Way Suite 2	Portland
Vaffeljernet	Palle Ibsen	Smagsløget 45	Århus
Wolski Zajazd	Zbyszek Piestrzeniewicz	ul. Filtrowa 68	Warszawa

To preserve space, the table above is a subset of the Customers table used in the example below.

Try it Yourself

To see how SQL works, you can copy the SQL statements below and paste them into the textarea, or you can make your own SQL statements.

SELECT * FROM customers

SELECT CompanyName, ContactName FROM customers

SELECT * FROM customers WHERE companyname LIKE ‘a%’

SELECT CompanyName, ContactName
FROM customers
WHERE CompanyName > ‘a’

When using SQL on text data, “alfred” is greater than “a” (like in a dictionary).

SELECT CompanyName, ContactName
FROM customers
WHERE CompanyName > ‘g’
AND ContactName > ‘g’

SELECT * FROM customers

SQL TOP

Deepanshu Mehta — Sun, 11 Oct 2009 17:32:35 +0000

The TOP Clause

The TOP clause is used to specify the number of records to return.

The TOP clause can be very useful on large tables with thousands of records. Returning a large number of records can impact on performance.

Note: Not all database systems support the TOP clause.

SQL Server Syntax

SELECT TOP number|percent column_name(s)
FROM table_name

SQL SELECT TOP Equivalent in MySQL and Oracle

MySQL Syntax

SELECT column_name(s)
FROM table_name
LIMIT number

Example

SELECT *
FROM Persons
LIMIT 5

Oracle Syntax

SELECT column_name(s)
FROM table_name
WHERE ROWNUM <= number

Example

SELECT *
FROM Persons
WHERE ROWNUM <=5

SQL TOP Example

The “Persons” table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger
4	Nilsen	Tom	Vingvn 23	Stavanger

Now we want to select only the two first records in the table above.

We use the following SELECT statement:

SELECT TOP 2 * FROM Persons

The result-set will look like this:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes

SQL TOP PERCENT Example

The “Persons” table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger
4	Nilsen	Tom	Vingvn 23	Stavanger

Now we want to select only 50% of the records in the table above.

We use the following SELECT statement:

SELECT TOP 50 PERCENT * FROM Persons

The result-set will look like this:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes