1. summary

SQL Server All functions that can be accomplished using the graphical interface can be utilized T-SQL To achieve ;
Use T-SQL In operation , And SQL Server All references to the communication are sent to the server by T-SQL Statement , It has nothing to do with the interface of the application ;
T-SQL Generated by multiple applications , It mainly includes 9 Parts of ：

1. Through the office production process ;
2. Use common language statements to determine the application of the data the user wants to query ;
3. Using a graphical user interface GUI Applications for , Enables the user to select the tables and columns that contain the data to view ;
4. Store its data with SQL Server Commercial applications in databases ; These applications can be applications from other vendors , It can also be an internally written application ;
5. Use osql And other utilities T-SQL Script ;
6. The development system implements an excuse for public database application （API, Such as ADO、OLE DB as well as ODBC） Created applications ;
7. from SQL Server The database submits the data Web page ;
8. Distributed database system , In this system, data is transferred from SQL Server Copy to various databases or execute distributed queries ;
9. Database warehouse , From online transactions （OLTP） Extract data from the system , And summarize the data for decision support analysis , Can be carried out in this warehouse ;

2. T-SQL The composition of language

T-SQL As a process language , In addition to establishing a connection with the database 、 In addition to processing data , It also has the elements of procedural language ： Batch command 、 identifier 、 System function 、 expression 、 Variable 、 data type 、 Operator 、 Flow control statement 、 notes 、 Keep keywords, etc ;
T-SQL Several process language elements supported ：

1. notes
   notes Is a text string that is not executed in the program code （ Also known as annotation ）;
   stay SQL Server in , Two types of comment characters can be used ： One is ANSI Standard annotators “–”, It is used for Single-line comments ; The other is with C Program annotation symbols with the same language , namely “/**/”, For multiline comments ;

2. Variable
   Variable Is an essential part of language ;T-SQL There are two forms of variables in a language , One is user-defined local variables , A local variable is an object that can have a specific data type , Its scope of action is limited to Inside the program ;
   local variable It can be used as a counter to calculate the number of times the loop is executed , Or control the number of times the fantasy is executed ; Using local variables can also save data values , For control flow statement testing and saving the value returned by the stored procedure , When a local variable is referenced, its name should be preceded by a flag “@”, And must first use DECLARE The command cannot be used until it is defined ;
   The other is provided by the system Global variables , The global variable is SQL Server Variables used inside the system , Its scope is not limited to a certain procedure , But any program can be called at any time ; Global variables usually store some SQL Server Configuration settings and statistics for ;
   The user can use global variables to test the set value or... Of the system in the program T-SQL State value after command execution ;

3. Operator
   Operator It's some symbols , It can be used to perform arithmetic operators 、 String connection 、 Assignment and in field 、 Constant 、 Compare between variables ;
   stay SQL Server in , There are six main categories of operators ： Arithmetic operator 、 Assignment operator 、 An operator 、 Comparison operator 、 Logical operators 、 String concatenation operator ;

T-SQL It is also an important part of process control statements ,T-SQL The program block is inseparable from process control ;

3. T-SQL Sentence structure

T-SQL The referenced syntax diagram is as follows ：

4. T-SQL sentence

utilize T-SQL , Users can create database devices 、 Databases and other database objects , Extract data from the database 、 Modifying data , Can also dynamically change SQL Server Settings in ;
By function ,SQL Language mainly includes ： Data manipulation statement 、 Data definition statement 、 Data control statement ;

1. Data operation statement （DML）： It mainly includes the query of object database data 、 Insert 、 Delete 、 Modify the operating ;
2. Data definition statement （DDL）： Used to define the structure and organization of the stored database , And the relationship between data items , As shown in the table 、 View 、 Triggers and stored procedures ;
3. Data control statement （DCL）： Including data storage control and integrity control , To prevent illegal users from using and destroying data ;

4.1 Data definition statement （DDL）

Data definition statements are used to perform data tasks , Create the database and various objects in the database 、 Delete 、 Modification and other operations ;
DDL The main sentences and functions are shown in the following table ：

4.2 Data operation statement （DML）

Data operation statements are used to operate various objects in the database 、 Retrieve and modify data ;
DML The main sentences and functions are shown in the following table ：

4.3 Data control statement （DCL）

Data control statements are used for security management , Determine which users can view or modify data in the database ,DCL The main sentences and functions are shown in the following table ：