SQL LECTURE 5

Lecture 5

The UPDATE Statement

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!

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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

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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 is used to delete records in a table.

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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!

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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

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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

or

DELETE * FROM table_name

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

Practice Makes the man perfect

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	Warszaw

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

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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'

Lecture 6

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

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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

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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

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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

The LIKE operator is used in a WHERE clause to search for a specified pattern in a column.

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The LIKE Operator

The LIKE operator is used to search for a specified pattern in a column.

SQL LIKE Syntax

SELECT column_name(s)

FROM table_name

WHERE column_name LIKE pattern

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LIKE 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 the persons living in a city that starts with "s" from the table above.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE City LIKE 's%'

The "%" sign can be used to define wildcards (missing letters in the pattern) both before and after the pattern.

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
3	Pettersen	Kari	Storgt 20	Stavanger

Next, we want to select the persons living in a city that ends with an "s" from the "Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE City LIKE '%s'

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

Next, we want to select the persons living in a city that contains the pattern "tav" from the "Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE City LIKE '%tav%'

The result-set will look like this:

P_Id	LastName	FirstName	Address	City
3	Pettersen	Kari	Storgt 20	Stavanger

It is also possible to select the persons living in a city that does NOT contain the pattern "tav" from the "Persons" table, by using the NOT keyword.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE City NOT LIKE '%tav%'

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 wildcards can be used when searching for data in a database.

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