Grammatical conventions
Identifier
Custom regular identifier (legally named)
-Starting with a letter or other language characters _, @, #
Split identifier (when there are spaces in the identifier, symbols that do not meet the basic rules, and the system reserves words, use a separator to identify)
Custom identifier
(1) The first character of the identifier must be one of the following characters.
The letters defined in the Unicode 2.0 standard include the Latin letters az and AZ, as well as alphabet characters in other languages. Underscore "_", symbol "@" or number symbol "#".
(2) The following characters of the identifier can be the following three types.
The letters defined in the Unicode 2.0 standard. Decimal numbers from Latin alphabet or scripts from other countries. "@" Sign, dollar sign "$", number sign "#" or underscore "_".
(3) The identifier is not allowed to be a reserved word of Transact-SQL.
(4) No spaces or other special characters are allowed
Delimiter
Identifier in double quotes ("")
Identifiers in square brackets ([])
• The identifier may not conform to the format rules of the identifier, such as [MR GZGLXT], there are spaces between MR and GZGLXT, but because square brackets are used, they are regarded as delimited identifiers. For example: [USER]
• Note: The number of characters contained in the regular identifier and the delimited identifier must be between 1 and 128. For a local temporary table, the identifier can have a maximum of 116 characters.
System reserved words
•
Similar to many other languages, SQL Server 2014 uses more than 180 reserved keywords (Reserved Keyword) to define, operate, or access databases and database objects.
•
These reserved keywords are part of the T-SQL syntax and are used to analyze and understand the T-SQL language, including DATABASE, CURSOR, CREATE, INSERT, BEGIN, etc.
• In
general, you cannot use these reserved keywords as object names or identifiers.
constant
•
Constants are values whose values remain unchanged during the running of the program.
•
1. String constant
–
'Hello'. 'Anyang City', N 'Henan Province' (Unicode constant)
•
2. Numeric constants
–
Integer constants 500, 899, 0x23 (hexadecimal number)
–
Real constants
•
Fixed point constant 3.6415
•
floating point constant 0.56E-3
•
3. Date time constant
–
‘1990/2/28’、’20-02-02’
•
4. Currency constant
–
56.45 (do not add ￥, $ in the table) ￥ 890, $ 893
variable
•
Variable refers to the amount that changes with the running of the program during the running of the program. It can save the query result and the return value of the stored procedure, and can also be used in the query.
•
Global variables
–
System-defined, no need to declare, use directly
–
Beginning with @@
–
For example: @@ error (error number of the last sql statement)
• Bureau of
Ministry of variables
–
User defined, scope is only inside the program
–
DECLARE variable name variable type
– At the
beginning
–
All variables are set to NULL after declaration
Global variables
Variable assignment
There are two ways to assign values to variables: use SET statements to directly assign values to variables and SELECT statements to select values in the table to assign values to variables. The syntax is as follows:
Format 1: Assignment using SET statement
SET variable name = expression
Format 2: Assignment using SELECT statement
SELECT variable name = expression [，… n]
Description:
The
expression can be any valid SQL expression;
A SELECT statement can assign values to multiple variables, and a SET statement can only assign values to one variable at a time.
Operator
Operator precedence
When the levels of the operators are different, the higher-level operators are operated first, and then the lower-level operators are operated. When the levels of operators in an expression are the same, the operations are generally performed from left to right. When there are parentheses in the expression, the expression in the parentheses should be evaluated first; if there are nested parentheses in the expression, the expression with the deepest nesting is evaluated first.
expression
•
Field expression after the table name field expression
• The
target expression after the select statement
•
Conditional expressions, conditional expressions after the where statement
Conditional expression
1. Compare sizes>
2. Specify the range between and
3. Collection in
4. Character matching like '<match string>'
5. Null value is null
6. Multiple conditions and or
Wildcard
•
Wildcards in the T-SQL language can replace one or more characters. Wildcards must be used in conjunction with the LIKE operator.
Comment
•
1. Single line comment
•
When the double hyphen "-" is used as a comment character, everything from the double hyphen to the end of the line is the comment content.
•
2. Multi-line comments
•
When using the comment character "/ * * /" as a multi-line comment character, everything from the beginning "/ *" to "* /" is a comment.
•
These comment characters can be used for multi-line text or code blocks.
Data Definition Language DDL
Data Definition Language DDL (Data Definition
Language): Realize the definition function of SQL. Used to define the logical structure of the database, including basic tables, views and indexes. The basic DDL includes three categories: definition, modification and deletion.
Data Manipulation Language DML
Realize the data manipulation function of SQL. Including two types of operations: data query and data update. Among them, data query refers to query, statistics, grouping, sorting and other operations on the data in the database; data update includes three operations of insert and delete modification.
Data Control Language DCL
Used to grant / cancel control rights to data objects to users, to authorize, recycle or deny access through GRANT, REVOKE and DENY. Database control refers to the security and integrity control of the database. SQL data control includes authorization of basic tables and views, description of integrity rules, and control statements such as transaction start and end.
Other basic sentences
•
Data statement
–
Variable declaration DECLARE variable name data type
–
The first character of the urine variable must be @; all variables are in the sound
Set the initial value to NULL after tomorrow
•
Data assignment
–
Assignment in two ways: SET and SELECT
–
SET @ variable = value (SET can only assign a variable)
–
SELECT @variable 1 = value 1, @variable 2 = value 2, ...
•
Data output
–
PRINT Urine variable (character type) or string
–
SELECT Urine variable AS custom column name
Data control statement
•
Flow control statements are statements used to control program execution and flow branching.
•
BEGIN……END
•
IF……ELSE
•
CASE
•
WHILE （CONTINUE、BREAK）
•
GOTO
•
WAITFOR
•
RETURN
BEGIN……END
）begin……end
•
A statement block composed of multiple Transact-SQL statements can be regarded as a statement.
–
BEGIN
–
{sql_statement |statement_block}
–
END
–
Among them, {sql_statement | statement_block} is any valid Transact-SQL statement or statement group defined by the statement block
if...else
case statement
1. Simple Case Statement
Case <condition judgment expression>
when condition judgment expression result 1 then <Transact-SQL command line or block
Statement>
when condition judgment expression result 2 then <Transact-SQL command line or block
Statement>
…
when condition judge expression result n then <Transact-SQL command line or block
Statement>
else <Transact-SQL command line or block statement>
end
2.Case search statement
Case
when
conditional expression 1 then <Transact-SQL command line or block statement>
when conditional expression 2 then <Transact-SQL command line or block statement>
…
when conditional expression n then <Transact-SQL command line or block statement>
else <Transact-SQL command line or block statement>
end
while statement
waitfor statement
• The
Waitfor statement is used to suspend the execution of the statement until the specified time point or specified time interval.
•
WAITFOR DELAY ’ time’|TIME ’ time’
•
Role:
•
(1) Delay execution at intervals
•
(2) Specify when to execute from, used to specify the trigger block, stored procedure and transaction execution time
return statement
• RETURN [integer expression]
• The RETURN statement is used to unconditionally exit from a query, stored procedure, or batch, and the statement will not execute after the RETURN statement
Row.
• (1) The stored procedure can return an integer value to the calling procedure or application. When used in a stored procedure, the RETURN statement cannot return a null value;
• (2) The system stored procedure returns a value of 0 to indicate success, and a non-zero value indicates failure.
