sql server中变量要先期说明后赋值:


说明有变量语法:declare @变量名 数据类型;例如:declare @num int;


set @num=value;   或   select @num=value;


select @num=字段名 from 表名 where ……


先是栽用法:set @num=1; 或set @num:=1;

亚种用法:select @num:=1; 或 select @num:=字段名 from 表名 where ……





User-Defined Variables

You can store a value in a user-defined variable in one statement and
then refer to it later in another statement. This enables you to pass
values from one statement to another.

User variables are written as @var_name, where the variable
name var_name consists of alphanumeric characters, “.”, “_”, and “$”. A
user variable name can contain other characters if you quote it as a
string or identifier (for example, @'my-var',@"my-var",
or @`my-var`).

User-defined variables are session-specific. A user variable defined by
one client cannot be seen or used by other clients. (Exception: A user
with access to the Performance
Schema user_variables_by_thread table
can see all user variables for all sessions.) All variables for a given
client session are automatically freed when that client exits.

User variable names are not case sensitive. Names have a maximum length
of 64 characters as of MySQL 5.7.5. (Length is not constrained before

One way to set a user-defined variable is by issuing
SET statement:

SET @var_name = expr [, @var_name = expr] ...

For SET,
either = or := can
be used as the assignment operator.

You can also assign a value to a user variable in statements other
than SET.
In this case, the assignment operator must
be := and
not = because
the latter is treated as the comparison
operator = in
non-SET statements:

mysql> SET @t1=1, @t2=2, @t3:=4;
mysql> SELECT @t1, @t2, @t3, @t4 := @t1+@t2+@t3;
| @t1  | @t2  | @t3  | @t4 := @t1+@t2+@t3 |
|    1 |    2 |    4 |                  7 |

User variables can be assigned a value from a limited set of data types:
integer, decimal, floating-point, binary or nonbinary string,
or NULL value. Assignment of decimal and real values does not preserve
the precision or scale of the value. A value of a type other than one of
the permissible types is converted to a permissible type. For example, a
value having a temporal or spatial data type is converted to a binary
string. A value having
the JSON data type
is converted to a string with a character set of utf8mb4 and a
collation of utf8mb4_bin.

If a user variable is assigned a nonbinary (character) string value, it
has the same character set and collation as the string. The coercibility
of user variables is implicit. (This is the same coercibility as for
table column values.)

Bit values assigned to user variables are treated as binary strings. To
assign a bit value as a number to a user variable,
use CAST() or +0:

mysql> SET @v1 = b'1000001';
mysql> SET @v2 = CAST(b'1000001' AS UNSIGNED), @v3 = b'1000001'+0;
mysql> SELECT @v1, @v2, @v3;
| @v1  | @v2  | @v3  |
| A    |   65 |   65 |

If the value of a user variable is selected in a result set, it is
returned to the client as a string.

If you refer to a variable that has not been initialized, it has a value
of NULL and a type of string.

User variables may be used in most contexts where expressions are
permitted. This does not currently include contexts that explicitly
require a literal value, such as in the LIMIT clause of
SELECT statement,
or the IGNORE N LINES clause of
aLOAD DATA statement.

As a general rule, other than
in SET statements,
you should never assign a value to a user variable and read the value
within the same statement. For example, to increment a variable, this is

SET @a = @a + 1;

For other statements, such
as SELECT, you
might get the results you expect, but this is not guaranteed. In the
following statement, you might think that MySQL will evaluate @a first
and then do an assignment second:

SELECT @a, @a:=@a+1, ...;

However, the order of evaluation for expressions involving user
variables is undefined.

Another issue with assigning a value to a variable and reading the value
within the same
non-SET statement
is that the default result type of a variable is based on its type at
the start of the statement. The following example illustrates this:

mysql> SET @a='test';
mysql> SELECT @a,(@a:=20) FROM tbl_name;

this SELECT statement,
MySQL reports to the client that column one is a string and converts all
accesses of @a to strings, even though @a is set to a number for the
second row. After
the SELECT statement
executes, @a is regarded as a number for the next statement.

To avoid problems with this behavior, either do not assign a value to
and read the value of the same variable within a single statement, or
else set the variable to 00.0, or '' to define its type before
you use it.

SELECT statement,
each select expression is evaluated only when sent to the client. This
means that in a HAVINGGROUP BY, or ORDER BY clause, referring to
a variable that is assigned a value in the select expression list
does not work as expected:

mysql> SELECT (@aa:=id) AS a, (@aa+3) AS b FROM tbl_name HAVING b=5;

The reference to b in the HAVING clause refers to an alias for an
expression in the select list that uses @aa. This does not work as
expected: @aa contains the value of id from the previous selected
row, not from the current row.

User variables are intended to provide data values. They cannot be used
directly in an SQL statement as an identifier or as part of an
identifier, such as in contexts where a table or database name is
expected, or as a reserved word such
asSELECT. This
is true even if the variable is quoted, as shown in the following

mysql> SELECT c1 FROM t;
| c1 |
|  0 |
|  1 |
2 rows in set (0.00 sec)

mysql> SET @col = "c1";
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @col FROM t;
| @col |
| c1   |
1 row in set (0.00 sec)

mysql> SELECT `@col` FROM t;
ERROR 1054 (42S22): Unknown column '@col' in 'field list'

mysql> SET @col = "`c1`";
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @col FROM t;
| @col |
| `c1` |
1 row in set (0.00 sec)

An exception to this principle that user variables cannot be used to
provide identifiers, is when you are constructing a string for use as a
prepared statement to execute later. In this case, user variables can be
used to provide any part of the statement. The following example
illustrates how this can be done:

mysql> SET @c = "c1";
Query OK, 0 rows affected (0.00 sec)

mysql> SET @s = CONCAT("SELECT ", @c, " FROM t");
Query OK, 0 rows affected (0.00 sec)

mysql> PREPARE stmt FROM @s;
Query OK, 0 rows affected (0.04 sec)
Statement prepared

mysql> EXECUTE stmt;
| c1 |
|  0 |
|  1 |
2 rows in set (0.00 sec)

Query OK, 0 rows affected (0.00 sec)