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SQL Tutorial w3schools.

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1 SQL Tutorial w3schools

2 Intro Intro

3 Sql intro SQL is a standard language for accessing and manipulating databases.

4 Sql intro SQL stands for Structured Query Language
What is SQL? SQL stands for Structured Query Language SQL lets you access and manipulate databases SQL became a standard of the American National Standards Institute (ANSI) in 1986, and of the International Organization for Standardization (ISO) in 1987

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

6 Sql intro SQL is a Standard - BUT.... Although SQL is an ANSI/ISO standard, there are 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!

7 Sql intro Using SQL in Your Web Site To build a web site that shows data from a database, you will need: An RDBMS database program (i.e. MS Access, SQL Server, MySQL) To use a server-side scripting language, like PHP or ASP To use SQL to get the data you want To use HTML / CSS to style the page

8 Sql intro RDBMS RDBMS stands for Relational Database Management System. RDBMS is the basis for SQL, and for all modern database systems such as 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. Look at the "Customers" table: SELECT * FROM Customers; Try it Yourself:

9 Sql intro RDBMS Continued Every table is broken up into smaller entities called fields. The fields in the Customers table consist of CustomerID, CustomerName, ContactName, Address, City, PostalCode and Country. A field is a column in a table that is designed to maintain specific information about every record in the table. A record, also called a row, is each individual entry that exists in a table. For example, there are 91 records in the above Customers table. A record is a horizontal entity in a table. A column is a vertical entity in a table that contains all information associated with a specific field in a table.

10 Syntax Syntax

11 Sql syntax Database Tables A 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. In this tutorial we will use the well-known Northwind sample database (included in MS Access and MS SQL Server).

12 Sql syntax Database Tables Continued
Below is a selection from the "Customers" table: The table above contains five records (one for each customer) and seven columns (CustomerID, CustomerName, ContactName, Address, City, PostalCode, and Country). CustomerID CustomerName ContactName Address City PostalCode Country 1 Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany 2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 05023 4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S Sweden

13 Sql syntax SQL Statements Most of the actions you need to perform on a database are done with SQL statements. The following SQL statement selects all the records in the "Customers" table: SELECT * FROM Customers; Try it Yourself: In this tutorial we will teach you all about the different SQL statements.

14 Sql syntax Keep in Mind That... SQL keywords are NOT case sensitive: select is the same as SELECT In this tutorial we will write all SQL keywords in upper-case.

15 Sql syntax 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. In this tutorial, we will use semicolon at the end of each SQL statement.

16 Sql syntax SELECT - extracts data from a database
Some of The Most Important SQL Commands 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 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

17 SELECT Select

18 Sql select The SQL SELECT Statement The SELECT statement is used to select data from a database. The data returned is stored in a result table, called the result-set. SELECT Syntax SELECT column1, column2, ... FROM table_name; Here, column1, column2, ... are the field names of the table you want to select data from. If you want to select all the fields available in the table, use the following syntax: SELECT * FROM table_name;

19 Sql select Demo Database Below is a selection from the "Customers" table in the Northwind sample database: CustomerID CustomerName ContactName Address City PostalCode Country 1 Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany 2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 05023 4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S Sweden

20 Sql select SELECT Column Example The following SQL statement selects the "CustomerName" and "City" columns from the "Customers" table: SELECT CustomerName, City FROM Customers; Try it Yourself:

21 Sql select Test Yourself With Exercises Start the Exercise

22 SELECT DISTINCT Select Distinct

23 Sql select Distinct The SQL SELECT DISTINCT Statement The SELECT DISTINCT statement is used to return only distinct (different) values. Inside a table, a column often contains many duplicate values; and sometimes you only want to list the different (distinct) values. SELECT DISTINCT Syntax SELECT DISTINCT column1, column2, ... FROM table_name;

24 Sql select Distinct Demo Database Below is a selection from the "Customers" table in the Northwind sample database: CustomerID CustomerName ContactName Address City PostalCode Country 1 Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany 2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 05023 4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S Sweden

25 Sql select Distinct SELECT Example The following SQL statement selects all (and duplicate) values from the "Country" column in the "Customers" table: SELECT Country FROM Customers; Try it Yourself: Now, let us use the DISTINCT keyword with the above SELECT statement and see the result.

26 Sql select Distinct SELECT DISTINCT Examples The following SQL statement selects only the DISTINCT values from the "Country" column in the "Customers" table: SELECT DISTINCT Country FROM Customers; Try it Yourself: The following SQL statement lists the number of different (distinct) customer countries: SELECT COUNT(DISTINCT Country) FROM Customers; Try it Yourself:

27 Sql select Distinct SELECT DISTINCT Examples Continued Here is the workaround for MS Access: SELECT Count(*) AS DistinctCountries FROM (SELECT DISTINCT Country FROM Customers); Try it Yourself: Note: The previous example will not work in Firefox and Microsoft Edge! Because COUNT(DISTINCT column_name) is not supported in Microsoft Access databases. Firefox and Microsoft Edge are using Microsoft Access in our examples.

28 Sql select Distinct Test Yourself With Exercises Start the Exercise

29 WHERE Where

30 Sql where The SQL WHERE Clause The WHERE clause is used to filter records. The WHERE clause is used to extract only those records that fulfill a specified condition. WHERE Syntax SELECT column1, column2, ... FROM table_name WHERE condition; Note: The WHERE clause is not only used in SELECT statement, it is also used in UPDATE, DELETE statement, etc.!

31 Sql where Demo Database Below is a selection from the "Customers" table in the Northwind sample database: CustomerID CustomerName ContactName Address City PostalCode Country 1 Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany 2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 05023 4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S Sweden

32 Sql where WHERE Clause Example The following SQL statement selects all the customers from the country "Mexico", in the "Customers" table: SELECT * FROM Customers WHERE Country='Mexico’; Try it Yourself:

33 Sql where Text Fields vs. Numeric Fields SQL requires single quotes around text values (most database systems will also allow double quotes). However, numeric fields should not be enclosed in quotes: SELECT * FROM Customers WHERE CustomerID=1; Try it Yourself:

34 Sql where The following operators can be used in the WHERE clause:
Operators in The WHERE Clause The following operators can be used in the WHERE clause: Operator Description = Equal <> Not equal. Note: In some versions of SQL this operator may be written as != > Greater than < Less than >= Greater than or equal <= Less than or equal BETWEEN Between a certain range LIKE Search for a pattern IN To specify multiple possible values for a column

35 Sql where Test Yourself With Exercises Start the Exercise

36 AND, Or, Not And, Or, Not

37 Sql and or not The SQL AND, OR and NOT Operators The WHERE clause can be combined with AND, OR, and NOT operators. The AND and OR operators are used to filter records based on more than one condition: The AND operator displays a record if all the conditions separated by AND are TRUE. The OR operator displays a record if any of the conditions separated by OR is TRUE. The NOT operator displays a record if the condition(s) is NOT TRUE.

38 Sql and or not The SQL AND, OR and NOT Operators Continued AND Syntax SELECT column1, column2, ... FROM table_name WHERE condition1 AND condition2 AND condition3 ...; OR Syntax SELECT column1, column2, ... FROM table_name WHERE condition1 OR condition2 OR condition3 ...; NOT Syntax SELECT column1, column2, ... FROM table_name WHERE NOT condition;

39 Sql and or not Demo Database Below is a selection from the "Customers" table in the Northwind sample database: CustomerID CustomerName ContactName Address City PostalCode Country 1 Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany 2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 05023 4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S Sweden

40 Sql and or not AND Example The following SQL statement selects all fields from "Customers" where country is "Germany" AND city is "Berlin": SELECT * FROM Customers WHERE Country='Germany' AND City='Berlin’; Try it Yourself:

41 Sql and or not OR Example The following SQL statement selects all fields from "Customers" where city is "Berlin" OR "München": SELECT * FROM Customers WHERE City='Berlin' OR City='München’; Try it Yourself:

42 Sql and or not NOT Example The following SQL statement selects all fields from "Customers" where country is NOT "Germany": SELECT * FROM Customers WHERE NOT Country='Germany’; Try it Yourself:

43 Sql and or not Combining AND, OR and NOT You can also combine the AND, OR and NOT operators. The following SQL statement selects all fields from "Customers" where country is "Germany" AND city must be "Berlin" OR "München" (use parenthesis to form complex expressions): SELECT * FROM Customers WHERE Country='Germany' AND (City='Berlin' OR City='München’); Try it Yourself: The following SQL statement selects all fields from "Customers" where country is NOT "Germany" and NOT "USA": SELECT * FROM Customers WHERE NOT Country='Germany' AND NOT Country='USA’; Try it Yourself:

44 Sql and or not Test Yourself With Exercises Start the Exercise

45 ORDER BY Order By

46 Sql order by The SQL ORDER BY Keyword The ORDER BY keyword is used to sort the result-set in ascending or descending order. The ORDER BY keyword sorts the records in ascending order by default. To sort the records in descending order, use the DESC keyword. ORDER BY Syntax SELECT column1, column2, ... FROM table_name ORDER BY column1, column2, ... ASC|DESC;

47 Sql order by Demo Database Below is a selection from the "Customers" table in the Northwind sample database: CustomerID CustomerName ContactName Address City PostalCode Country 1 Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany 2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 05023 4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S Sweden

48 Sql order by ORDER BY Example The following SQL statement selects all customers from the "Customers" table, sorted by the "Country" column: SELECT * FROM Customers ORDER BY Country; Try it Yourself:

49 Sql order by ORDER BY DESC Example The following SQL statement selects all customers from the "Customers" table, sorted DESCENDING by the "Country" column: SELECT * FROM Customers ORDER BY Country DESC; Try it Yourself:

50 Sql order by ORDER BY Several Columns Example The following SQL statement selects all customers from the "Customers" table, sorted by the "Country" and the "CustomerName" column. This means that it orders by Country, but if some rows have the same Country, it orders them by CustomerName: SELECT * FROM Customers ORDER BY Country, CustomerName; Try it Yourself:

51 Sql order by ORDER BY Several Columns Example 2 The following SQL statement selects all customers from the "Customers" table, sorted ascending by the "Country" and descending by the "CustomerName" column: SELECT * FROM Customers ORDER BY Country ASC, CustomerName DESC; Try it Yourself:

52 Sql order by Test Yourself With Exercises Start the Exercise

53 INSERT INTO Insert Into

54 Sql Insert into The SQL INSERT INTO Statement The INSERT INTO statement is used to insert new records in a table. INSERT INTO Syntax It is possible to write the INSERT INTO statement in two ways. The first way specifies both the column names and the values to be inserted: INSERT INTO table_name (column1, column2, column3, ...) VALUES (value1, value2, value3, ...); If you are adding values for all the columns of the table, you do not need to specify the column names in the SQL query. However, make sure the order of the values is in the same order as the columns in the table. The INSERT INTO syntax would be as follows: INSERT INTO table_name VALUES (value1, value2, value3, ...);

55 Sql Insert into Demo Database Below is a selection from the "Customers" table in the Northwind sample database: CustomerID CustomerName ContactName Address City PostalCode Country 89 White Clover Markets Karl Jablonski th Ave. S. Suite 3B Seattle 98128 USA 90 Wilman Kala Matti Karttunen Keskuskatu 45 Helsinki 21240 Finland 91 Wolski Zbyszek ul. Filtrowa 68 Walla 01-012 Poland

56 Sql Insert into INSERT INTO Example The following SQL statement inserts a new record in the "Customers" table: INSERT INTO Customers (CustomerName, ContactName, Address, City, PostalCode, Country) VALUES ('Cardinal', 'Tom B. Erichsen', 'Skagen 21', 'Stavanger', '4006', 'Norway’); Try it Yourself:

57 Sql Insert into INSERT INTO Example Continued The selection from the "Customers" table will now look like this: CustomerID CustomerName ContactName Address City PostalCode Country 89 White Clover Markets Karl Jablonski th Ave. S. Suite 3B Seattle 98128 USA 90 Wilman Kala Matti Karttunen Keskuskatu 45 Helsinki 21240 Finland 91 Wolski Zbyszek ul. Filtrowa 68 Walla 01-012 Poland 92 Cardinal Tom B. Erichsen Skagen 21 Stavanger 4006 Norway

58 Sql Insert into INSERT INTO Example Continued
Did you notice that we did not insert any number into the CustomerID field? The CustomerID column is an auto-increment field and will be generated automatically when a new record is inserted into the table.

59 Sql Insert into Insert Data Only in Specified Columns It is also possible to only insert data in specific columns. The following SQL statement will insert a new record, but only insert data in the "CustomerName", "City", and "Country" columns (CustomerID will be updated automatically): INSERT INTO Customers (CustomerName, City, Country) VALUES ('Cardinal', 'Stavanger', 'Norway’); Try it Yourself:

60 Sql Insert into Insert Data Only in Specified Columns Continued The selection from the "Customers" table will now look like this: CustomerID CustomerName ContactName Address City PostalCode Country 89 White Clover Markets Karl Jablonski th Ave. S. Suite 3B Seattle 98128 USA 90 Wilman Kala Matti Karttunen Keskuskatu 45 Helsinki 21240 Finland 91 Wolski Zbyszek ul. Filtrowa 68 Walla 01-012 Poland 92 Cardinal null null  Stavanger Norway

61 Sql Insert into Test Yourself With Exercises Start the Exercise

62 NULL Null Values

63 Sql Null values What is a NULL Value? A field with a NULL value is a field with no value. If a field in a table is optional, it is possible to insert a new record or update a record without adding a value to this field. Then, the field will be saved with a NULL value. Note: A NULL value is different from a zero value or a field that contains spaces. A field with a NULL value is one that has been left blank during record creation!

64 Sql Null values How to Test for NULL Values? It is not possible to test for NULL values with comparison operators, such as =, <, or <>. We will have to use the IS NULL and IS NOT NULL operators instead. IS NULL Syntax SELECT column_names FROM table_name WHERE column_name IS NULL; IS NOT NULL Syntax SELECT column_names FROM table_name WHERE column_name IS NOT NULL;

65 Sql Null values Demo Database Below is a selection from the "Customers" table in the Northwind sample database: CustomerID CustomerName ContactName Address City PostalCode Country 1 Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany 2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 05023 4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S Sweden

66 Tip: Always use IS NULL to look for NULL values.
Sql Null values The IS NULL Operator The IS NULL operator is used to test for empty values (NULL values). The following SQL lists all customers with a NULL value in the "Address" field: SELECT CustomerName, ContactName, Address FROM Customers WHERE Address IS NULL; Try it Yourself: Tip: Always use IS NULL to look for NULL values.

67 Sql Null values The IS NOT NULL Operator The IS NOT NULL operator is used to test for non-empty values (NOT NULL values). The following SQL lists all customers with a value in the "Address" field: SELECT CustomerName, ContactName, Address FROM Customers WHERE Address IS NOT NULL; Try it Yourself:

68 Sql Null values Test Yourself With Exercises Start the Exercise

69 UPDATE Update

70 Sql update The SQL UPDATE Statement The UPDATE statement is used to modify the existing records in a table. UPDATE Syntax UPDATE table_name SET column1 = value1, column2 = value2, ... WHERE condition; Note: Be careful when updating records in a table! Notice the WHERE clause in the UPDATE statement. The WHERE clause specifies which record(s) that should be updated. If you omit the WHERE clause, all records in the table will be updated!

71 Sql update Demo Database Below is a selection from the "Customers" table in the Northwind sample database: CustomerID CustomerName ContactName Address City PostalCode Country 1 Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany 2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 05023 4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S Sweden

72 Sql update UPDATE Table The following SQL statement updates the first customer (CustomerID = 1) with a new contact person and a new city. UPDATE Customers SET ContactName = 'Alfred Schmidt', City= 'Frankfurt' WHERE CustomerID = 1; Try it Yourself:

73 Sql update UPDATE Table Continued The selection from the "Customers" table will now look like this: CustomerID CustomerName ContactName Address City PostalCode Country 1 Alfreds Futterkiste Alfred Schmidt Obere Str. 57 Frankfurt 12209 Germany 2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 05023 4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S Sweden

74 Sql update UPDATE Multiple Records It is the WHERE clause that determines how many records that will be updated. The following SQL statement will update the contactname to "Juan" for all records where country is "Mexico": UPDATE Customers SET ContactName='Juan' WHERE Country='Mexico’; Try it Yourself:

75 Sql update UPDATE Multiple Records Continued The selection from the "Customers" table will now look like this: CustomerID CustomerName ContactName Address City PostalCode Country 1 Alfreds Futterkiste Alfred Schmidt Obere Str. 57 Frankfurt 12209 Germany 2 Ana Trujillo Emparedados y helados Juan Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Mataderos 2312 05023 4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S Sweden

76 Sql update Update Warning! UPDATE Customers SET ContactName='Juan’; Try it Yourself: Be careful when updating records. If you omit the WHERE clause, ALL records will be updated!

77 Sql update Update Warning! Continued The selection from the "Customers" table will now look like this: CustomerID CustomerName ContactName Address City PostalCode Country 1 Alfreds Futterkiste Juan Obere Str. 57 Frankfurt 12209 Germany 2 Ana Trujillo Emparedados y helados Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Mataderos 2312 05023 4 Around the Horn 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Berguvsvägen 8 Luleå S Sweden

78 Sql update Test Yourself With Exercises Start the Exercise

79 DELETE Delete

80 Sql Delete The SQL DELETE Statement The DELETE statement is used to delete existing records in a table. DELETE Syntax DELETE FROM table_name WHERE condition; Note: Be careful when deleting records in a table! Notice the WHERE clause in the DELETE statement. The WHERE clause specifies which record(s) should be deleted. If you omit the WHERE clause, all records in the table will be deleted!

81 Sql Delete Demo Database Below is a selection from the "Customers" table in the Northwind sample database: CustomerID CustomerName ContactName Address City PostalCode Country 1 Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany 2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 05023 4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S Sweden

82 Sql Delete SQL DELETE Example The following SQL statement deletes the customer "Alfreds Futterkiste" from the "Customers" table: DELETE FROM Customers WHERE CustomerName='Alfreds Futterkiste’; Try it Yourself:

83 Sql Delete The "Customers" table will now look like this:
SQL DELETE Example Continued The "Customers" table will now look like this: CustomerID CustomerName ContactName Address City PostalCode Country 2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 05023 4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S Sweden

84 Sql Delete Delete All Records 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; The following SQL statement deletes all rows in the "Customers" table, without deleting the table: DELETE FROM Customers; Try it Yourself:

85 Sql Delete Test Yourself With Exercises Start the Exercise

86 SELECT TOP Select Top

87 Sql Select top The SQL SELECT TOP Clause The SELECT TOP clause is used to specify the number of records to return. The SELECT TOP clause is useful on large tables with thousands of records. Returning a large number of records can impact on performance. SQL Server / MS Access Syntax: SELECT TOP number|percent column_name(s) FROM table_name WHERE condition; Note: Not all database systems support the SELECT TOP clause. MySQL supports the LIMIT clause to select a limited number of records, while Oracle uses ROWNUM.

88 Sql Select top The SQL SELECT TOP Clause Continued MySQL Syntax: SELECT column_name(s) FROM table_name WHERE condition LIMIT number; Oracle Syntax: SELECT column_name(s) FROM table_name WHERE ROWNUM <= number;

89 Sql Select top Demo Database Below is a selection from the "Customers" table in the Northwind sample database: CustomerID CustomerName ContactName Address City PostalCode Country 1 Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany 2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico 3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 05023 4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK 5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S Sweden

90 Sql Select top SQL TOP, LIMIT and ROWNUM Examples The following SQL statement selects the first three records from the "Customers" table: SELECT TOP 3 * FROM Customers; Try it Yourself: The following SQL statement shows the equivalent example using the LIMIT clause: SELECT * FROM Customers LIMIT 3; Try it Yourself:

91 Sql Select top SQL TOP, LIMIT and ROWNUM Examples Continued The following SQL statement shows the equivalent example using ROWNUM: SELECT * FROM Customers WHERE ROWNUM <= 3;

92 Sql Select top SQL TOP PERCENT Example The following SQL statement selects the first 50% of the records from the "Customers" table: SELECT TOP 50 PERCENT * FROM Customers; Try it Yourself:

93 Sql Select top ADD a WHERE CLAUSE The following SQL statement selects the first three records from the "Customers" table, where the country is "Germany": SELECT TOP 3 * FROM Customers WHERE Country='Germany’; Try it Yourself: The following SQL statement shows the equivalent example using the LIMIT clause: SELECT * FROM Customers WHERE Country='Germany' LIMIT 3; Try it Yourself:

94 Sql Select top ADD a WHERE CLAUSE Continued The following SQL statement shows the equivalent example using ROWNUM: SELECT * FROM Customers WHERE Country='Germany' AND ROWNUM <= 3;


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