SQL 变量

1、定义

变量分为三种：普通变量、表变量、游标。

变量定义后值为 NULL，在一个 GO 语句后失效。

DECLARE
   {    { @local_variable data_type }
      | { @cursor_variable_name CURSOR }
      | { @table_variable_name <table_type_definition> }
   } [ ,...n]

<table_type_definition> ::=

TABLE
   (
      { <column_definition> | <table_constraint> }
      [ ,...]
   )

<column_definition> ::=

column_name scalar_data_type
   [ COLLATE collation_ name ]
   [
      [ DEFAULT constant_expression ]
      | IDENTITY [ ( seed, increment ) ]
   ]
   [ ROWGUIDCOL ]
   [ <column_constraint> ]

<column_constraint> ::=

{    [ NULL | NOT NULL ]
   | [ PRIMARY KEY | UNIQUE ]
   | CHECK ( logical_expression )
}

<table_constraint> ::=

{
{ PRIMARY KEY | UNIQUE } ( column_name [ ,...] )
| CHECK ( search_condition )
}

2、赋值

SET @var_name = 100

SELECT @total = SUM(num) FROM table