[Note] database basics 03, note database 03

[Note] database basics 03, note database 03
Basic SQL database syntax

 

Basic statement usage and database basics.

Unoin usage

Each result set must have the same number of columns and have the same type.

Eg:

Select Name from Student

Union

Select Age from Student

<If all is added after union, the number of duplicates is not removed.>

A data type is a type of data attribute, indicating the type of information represented by the data. Any computer language defines its own data type. Of course, different programming languages have different characteristics. The types and names of the defined data types are more or less different. SQLServer provides 25 data types:
· Binary [(n)]
· Varbinary [(n)]
· Char [(n)]
· Varchar [(n)]
· Nchar [(n)]
· Nvarchar [(n)]
· Datetime
· Smalldatetime
· Decimal [(p [, s])]
· Numeric [(p [, s])]
· Float [(n)]
· Real
· Int
· Smallint
· Tinyint
· Money
· Smallmoney
· Bit
· Cursor
· Sysname
· Timestamp
· Uniqueidentifier
· Text
· Image
· Ntext
　　(1) binary data type
Binary data includesBinary, Varbinary, and Image
The Binary data type can be either fixed-length (Binary) or variable-length.
Binary [(n)] is a fixed n-bit Binary data. The value range of n is from 1 to 8000. The memory size is n + 4 bytes.
Varbinary [(n)] is a binary data with n-bit varying length. The value range of n is from 1 to 8000. The memory size is n + 4 bytes, not n Bytes.
The data stored in the Image data type is stored as a bit string, which is not explained by SQL Server and must be explained by the application. For example, an application can store data in the Image data type in BMP, TIEF, GIF, and JPEG formats.
　　(2) character data type
The types of character data includeChar, Varchar, and Text
Character data is composed of any combination of letters, symbols, and numbers.
Varchar is a variable-length data with a length of no more than 8 KB. Char is a fixed-length character data with a maximum length of 8 KB. ASCII data larger than 8 KB can be stored as Text data. For example, because all Html documents are ASCII characters and generally exceed 8 kb in length, these documents can be stored in SQL Server as Text data.
　　(3) Unicode Data Type
Unicode data types includeNchar, Nvarchar, and Ntext
In Microsoft SQL Server, traditional non-Unicode data types allow the use of characters defined by specific character sets. You can select a character set during SQL Server installation. The Unicode data type allows columns to store any characters defined by the Unicode standard. The Unicode Standard contains all characters defined in various character sets. When Unicode data types are used, the odds are two times the bytes used by non-Unicode data types.
In SQL Server, Unicode data is stored as Nchar, Nvarchar, and Ntext data. Columns stored with this character type can store characters in multiple character sets. When the length of a column changes, the Nvarchar character type should be used. At this time, a maximum of 4000 characters can be stored. When the column length remains unchanged, the Nchar character type should be used. Similarly, a maximum of 4000 characters can be stored. When the Ntext data type is used, this column can store more than 4000 characters.
　　(4) Date and Time Data Types
Date and Time data types includeDatetime and SmalldatetimeTwo Types
The date and time data types are composed of valid date and time. For example, valid date and time data includes "4/01/98 12: 15: 00: 00: 00 PM" and "1: 28: 29: 15: 01 AM 8/17/98 ". The previous data type is before the date, the time is before the last data type is before the time, and the date is after. In Microsoft SQL Server, the date and time data types include Datetime and Smalldatetime. The stored date range starts from January 1, January 1, 1753, end on April 9, December 31, 9999 (each value requires 8 storage bytes ). When the Smalldatetime data type is used, the stored date range is from January 1, January 1, 1900 to January 1, December 31, 2079 (each value requires four storage bytes ).
The date format can be set. The command to set the date format is as follows:
Set DateFormat {format | @ format _ var |
Where, format | @ format_var is the date order. Valid parameters include MDY, DMY, YMD, YDM, MYD, and DYM. By default, the date format is MDY.
For example, after you run Set DateFormat YMD, the date format is in the format of year, month, and day. After you run Set DateFormat DMY, the date format is in the format of day, month, and year.
　　(5) Digital Data Types
Numeric data only contains numbers. Numeric data types include positive and negative numbers, decimals (floating point numbers), and Integers
Integers are composed of positive integers and negative integers, such as 39, 25, 0-2, and 33967. In Micrsoft SQL Server, the integer storage data type isInt, Smallint, and Tinyint. The range of Int data type stored data is greater than that of Smallint data type stored data, while that of Smallint data type stored data is greater than that of Tinyint data type stored data. The range for storing data using the Int data type is from-2 147 483 648 147 to 2 483 647 (each value requires 4 bytes of storage space ). When the Smallint data type is used, the data storage range is from-32 768 to 32 767 (each value requires 2 bytes of storage space ). When the Tinyint data type is used, the data storage range is from 0 to 255 (each value requires 1 byte storage space ).
The exact Decimal data types in SQL Server are Decimal and Numeric. The storage space occupied by such data is determined based on the number of digits after the number of digits of the data.
In SQL Server, the Data Types of approximate decimal data are Float and Real. For example, the 1/3 score is recorded. 3333333. It can be accurately expressed when an approximate data type is used. Therefore, the data retrieved from the system may be different from the data stored in this column.
　　(6) currency data indicates the number of positive or negative currencies.
In Microsoft SQL Server, the data type of currency data isMoney and Smallmoney
The Money data type requires 8 storage bytes, And the Smallmoney data type requires 4 Storage bytes.
　　(7) Special Data Types
Special data types include data types not mentioned previously. There are three special data types:Timestamp, Bit, and Uniqueidentifier.
Timestamp is used to indicate the sequence of SQL Server activities, in the binary projection format. The Timestamp data has no relationship with the inserted data or the date and time.
Bit is composed of 1 or 0. The Bit data type is used when it indicates true or false, ON, or OFF. For example, you can query whether a client request for each access can be stored in a column of this data type.
Uniqueidentifier consists of 16-byte hexadecimal numbers, indicating a globally unique one. GUID is useful when the record rows in a table must be unique. For example, using this data type in the Customer ID column can distinguish different customers