W3SCHOOLS SQL EBOOK
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18+ Best Online Resources for Learning SQL and Database Concepts (2019 revision)
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How you found the violation and any other useful info. The following SQL statement will update the contactname to "Juan" for all records where country is "Mexico": This means that the table structure, attributes, and indexes will be intact: The MAX function returns the largest value of the selected column. The AVG function returns the average value of a numeric column.
The SUM function returns the total sum of a numeric column. There are two wildcards used in conjunction with the LIKE operator: MS Access uses a question mark? The percent sign and the underscore can also be used in combinations!
The IN operator is a shorthand for multiple OR conditions. In addition; do not show products with a CategoryID of 1,2, or 3: Aliases are often used to make column names more readable.
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An alias only exists for the duration of the query. And a selection from the "Orders" table: It requires double quotation marks or square brackets if the alias name contains spaces: We use the "Customers" and "Orders" tables, and give them the table aliases of "c" and "o" respectively Here we use aliases to make the SQL shorter: CustomerID; Aliases can be useful when: There are more than one table involved in a query Functions are used in the query Column names are big or not very readable Two or more columns are combined together Content Downloaded from www.
Let's look at a selection from the "Orders" table: CustomerID; Content Downloaded from www. Return all records when there is a match in either left or right table Content Downloaded from www. Below is a selection from the "Orders" table: The following SQL statement selects all orders with customer information: CustomerID; Note: The following SQL statement selects all orders with customer and shipper information: ShipperID ; Content Downloaded from www.
In this tutorial we will use the well-known Northwind sample database. CustomerName; Note: OrderID; Note: CustomerName; A selection from the result set may look like this: The following SQL statement matches customers that are from the same city: City; Content Downloaded from www.
Arbor The following SQL statement selects all the different cities only distinct values from "Customers" and "Suppliers": The following SQL statement selects all cities duplicate values also from "Customers" and "Suppliers": Only include countries with more than 5 customers: The following SQL statement lists the employees that have registered more than 10 orders: Copy all columns into a new table: The following SQL statement creates a backup copy of Customers: CustomerID; Tip: The empty "Persons" table will now look like this: The following SQL statement drops the existing table "Shippers": To add a column in a table, use the following syntax: We use the following SQL statement: The "Persons" table will now look like this: A table can have only one primary key, which may consist of single or multiple fields.
Look at the following two tables: The default value will be added to all new records IF no other value is specified. Creates an index on a table.
Duplicate values are allowed: Duplicate values are not allowed: MS Access: To insert a new record into the "Persons" table, we will NOT have to specify a value for the "ID" column a unique value will be added automatically: The view is created with the following SQL: Note that this view selects its data from another view called "Product Sales for ": Let's see the total sale only for the category "Beverages": We will update the view with the following SQL: Since SQL statements are text only, it is easy, with a little piece of computer code, to dynamically change SQL statements to provide the user with selected data: If there is nothing to prevent a user from entering "wrong" input, the user can enter some "smart" input like this: The SQL statement above is much the same as this: Here is a common construction, used to verify user login to a web site: User Name: The code at the server will create a valid SQL statement like this: If we had the following server code: User id: The code at the server would create a valid SQL statement like this: One of the top website vulnerabilities.
Oracle is also a popular database software for database-driven web sites with high traffic. Oracle is a very powerful, robust and full featured SQL database system. MySQL is also a popular database software for web sites. When a web site requires only a simple database, Microsoft Access can be a solution.
SQL aggregate functions return a single value, calculated from values in a column. Below is a selection from the "Products" table: The field to round. Specifies the number of decimals to be returned. Below is a selection from the "OrderDetails" table: The field to extract characters from start Required. Specifies the starting position starts at 1 length Optional. If omitted, the MID function returns the rest of the text Note: The date types are chosen for a column when you create a new table in your database!
For an overview of all data types available, go to our complete Data Types reference. You can compare two dates easily if there is no time component involved!
Assume we have the following "Orders" table: This is because the query is looking only for dates with no time portion. Now we want to insert a record into the "Orders" table: DATE date Where date is a valid date expression. OrderId OrderPayDate 1 OrderId SubtractDate 1 Only the date parts of the values are used in the calculation. DiffDate -1 Content Downloaded from www. The formats that can be used are: CurrentDateTime The time part above goes all the way to milliseconds.
DiffDate 61 Now we want to get the number of days between two dates notice that the second date is "earlier" than the first date, and will result in a negative number. DiffDate Content Downloaded from www.
The field to be formatted. Specifies the format. In this case we want NULL values to be zero. However, we can use the NVL function to achieve the same result: Each column in a database table is required to have a name and a data type.
The following table lists the general data types in SQL: Precision 19 Content Downloaded from www. The following table shows some of the common names of data types between the various database platforms: Data type Description Storage Text Use for text or combinations of text and numbers.
However, they are searchable Byte Allows whole numbers from 0 to 1 byte Integer Allows whole numbers between , and 32, 2 bytes Long Allows whole numbers between -2,,, and 4 bytes 2,,, Single Single precision floating-point.
Will handle most decimals 4 bytes Double Double precision floating-point.
Will handle most 8 bytes decimals Currency Use for currency. Holds up to 4,,, bytes of data ENUM x,y,z,etc. The values are sorted in the order you enter them. The maximum number of digits may be specified in parenthesis INT size to normal. The maximum number of digits may be specified in parenthesis BIGINT size to normal.
The maximum number of digits may be specified in parenthesis FLOAT size,d A small number with a floating decimal point. The maximum number of digits to the right of the decimal point is specified in the d parameter DOUBLE size,d A large number with a floating decimal point.
SS Note: Maximum 8, Defined width characters varchar n Variable width character string. Maximum 4, Defined width characters x2 nvarchar Variable width Unicode string. Maximum 4, characters nvarchar max Variable width Unicode string. Maximum ,, characters ntext Variable width Unicode string. Maximum 8, bytes varbinary Variable width binary string. Maximum 8, bytes varbinary max Variable width binary string.
Maximum 2GB image Variable width binary string. Maximum 2GB Content Downloaded from www.
Default value is 0 numeric p,s Fixed precision and scale numbers. Default value is 0 smallmoney Monetary data from , From January 1, to December 3 bytes 31, time Store a time only to an accuracy of nanoseconds bytes datetimeoffset The same as datetime2 with the addition of a time zone offset bytes timestamp Stores a unique number that gets updated every time a row gets created or modified.
Each table may have only one timestamp variable Content Downloaded from www. Maximum 2GB cursor Stores a reference to a cursor used for database operations table Stores a result-set for later processing Content Downloaded from www. Yohaness Be Bk. Rhazy la Bestia.
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Julian Manongdo. This command cannot be used to list all employees in Norway and USA. In the example above we have two employees with equal names, and only one of them is listed. Example This example demonstrates how you can create a table named "Person", with four columns.
The table below contains the most common data types in SQL: Data Type integer size int size smallint size tinyint size decimal size,d numeric size,d char size varchar size Description Hold integers only.
The maximum number of digits are specified in parenthesis. Hold numbers with fractions. The maximum number of digits are specified in "size". The maximum number of digits to the right of the decimal is specified in "d". Holds a fixed length string can contain letters, numbers, and special characters.
The fixed size is specified in parenthesis. Holds a variable length string can contain letters, numbers, and special characters. The maximum size is specified in parenthesis. Create Index Indices are created in an existing table to locate rows more quickly and efficiently.
It is possible to create an index on one or more columns of a table, and each index is given a name. The users cannot see the indexes, they are just used to speed up queries. Updating a table containing indexes takes more time than updating a table without, this is because the indexes also need an update.
So, it is a good idea to create indexes only on columns that are often used for a search. A Unique Index Creates a unique index on a table. A unique index means that two rows cannot have the same index value. A Simple Index Creates a simple index on a table.
Delete a Table or Database To delete a table the table structure, attributes, and indexes will also be deleted:. Truncate a Table What if we only want to get rid of the data inside a table, and not the table itself? The basic types of functions are:. Aggregate functions Aggregate functions operate against a collection of values, but return a single value. Scalar functions Scalar functions operate against a single value, and return a single value based on the input value.
Amount The above code is invalid because the column returned is not part of an aggregate. Make a Backup Copy The following example makes a backup copy of the "Persons" table:. What is a View?
A view contains rows and columns, just like a real table.
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The fields in a view are fields from one or more real tables in the database. The database design and structure will NOT be affected by the functions, where, or join statements in a view.
The database does not store the view data! Using Views A view could be used from inside a query, a stored procedure, or from inside another view. By adding functions, joins, etc. The sample database Northwind has some views installed by default.
The view "Current Product List" lists all active products products that are not discontinued from the Products table. The view is created with the following SQL:. Note that this view select its data from another view called "Product Sales for ":. Now we want to see the total sale only for the category "Beverages":. Flag for inappropriate content.
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Related titles. Jump to Page. Search inside document. A query like this:The time part above goes all the way to milliseconds. It's worth mentioning that you can download the whole tutorial as a PDF file to get back to it at any time, even when offline. You find also similar courses to SQL Server. Let's see the total sale only for the category "Beverages": However, we can use the NVL function to achieve the same result:.
The following example creates a table called "Persons" that contains five columns: That is why You should follow the syntax of these realizations when building queries.
Let's illustrate the foreign key with an example. The view is created with the following SQL: INT size to normal.
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