T-SQL和MySQL的一些常用语法的区别

Posted 2020-09-07 金泽夕（Santiago）

本文将主要列出mysql与SqlServer的SQL语句的一些常用语法的不同之处，且以常用的存储过程的相关内容为主。
1. 标识符限定符 
SqlServer
[]
MySql
``
2. 字符串相加
 SqlServer
直接用 +
MySql
concat()
3. isnull()
 SqlServer
isnull()
MySql
ifnull()
注意：MySql也有isnull()函数，但意义不一样
4. getdate()
SqlServer
getdate()
MySql
now()
5. newid()
SqlServer

newid()
MySql
uuid()

6. @@ROWCOUNT
SqlServer
@@ROWCOUNT
MySql
row_count()
注意：MySql的这个函数仅对于update, insert, delete有效
7. SCOPE_IDENTITY()
SqlServer
SCOPE_IDENTITY()
MySql
last_insert_id()
8. if ... else ...
SqlServer

IF Boolean_expression
{ sql_statement | statement_block }

[ ELSE

{ sql_statement | statement_block } ]

 

-- 若要定义语句块，请使用控制流关键字 BEGIN 和 END。

 

MySql

IF search_condition THEN statement_list

[ELSEIF search_condition THEN statement_list] ... [ELSE statement_list]

END IF

注意：对于MySql来说，then, end if是必须的。类似的还有其它的流程控制语句，这里就不一一列出。

9. declare

其实，SqlServer和MySql都有这个语句，用于定义变量，但差别在于：在MySql中，DECLARE仅被用在BEGIN ... END复合语句里，并且必须在复合语句的开头，在任何其它语句之前。这个要求在写游标时，会感觉很BT.

10. 游标的写法

 

SqlServer

declare @tempShoppingCart table (ProductId int, Quantity int)

insert into @tempShoppingCart (ProductId, Quantity)

select ProductId, Quantity from ShoppingCart where UserGuid = @UserGuid

 

 

declare @productId int

declare @quantity int

declare tempCartCursor cursor for

select ProductId, Quantity from @tempShoppingCart

 

open tempCartCursor

fetch next from tempCartCursor into @productId, @quantity

while @@FETCH_STATUS = 0

begin

update Product set SellCount = SellCount + @quantity where productId = @productId

 

fetch next from tempCartCursor into @productId, @quantity end

 

close tempCartCursor

deallocate tempCartCursor

 

 

MySql

declare m_done int default 0;

declare m_sectionId int;

declare m_newsId int;

 

declare _cursor_SN cursor for select sectionid, newsid from _temp_SN; declare continue handler for not found set m_done = 1;

 

create temporary table _temp_SN

select sectionid, newsid from SectionNews group by sectionid, newsid having count(*) > 1;

 

open _cursor_SN;

while( m_done = 0 ) do

fetch _cursor_SN into m_sectionId, m_newsId;

 

if( m_done = 0 ) then

-- 具体的处理逻辑

end if;

end while;

close _cursor_SN;

drop table _temp_SN;

注意：为了提高性能，通常在表变量上打开游标，不要直接在数据表上打开游标。

11. 分页的处理

 

SqlServer

create procedure GetProductByCategoryId(

@CategoryID int,

@PageIndex int = 0,

@PageSize int = 20,

@TotalRecords int output

)

as

begin

 

declare @ResultTable table

(

RowIndex int,

ProductID int,

ProductName nvarchar(50),

CategoryID int,

Unit nvarchar(10),

UnitPrice money,

Quantity int

);

 

insert into @ResultTable

select row_number() over (order by ProductID asc) as RowIndex, p.ProductID, p.ProductName, p.CategoryID, p.Unit, p.UnitPrice, p.Quantity

from Products as p

where CategoryID = @CategoryID;

 

select @TotalRecords = count(*) from @ResultTable;

 

select *

from @ResultTable

where RowIndex > (@PageSize * @PageIndex) and RowIndex <= (@PageSize * (@PageIndex+1));

 

end;

当然，SqlServer中并不只有这一种写法，只是这种写法是比较常见而已。

 

MySql

create procedure GetProductsByCategoryId(

in _categoryId int,

in _pageIndex int,

in _pageSize int,

out _totalRecCount int

)

begin

 

set @categoryId = _categoryId;

set @startRow = _pageIndex * _pageSize;

set @pageSize = _pageSize;

 

prepare PageSql from

‘select sql_calc_found_rows * from product where categoryId

= ? order by ProductId desc limit ?, ?‘;

execute PageSql using @categoryId, @startRow, @pageSize;

deallocate prepare PageSql;

set _totalRecCount = found_rows();

 


end

 

以上只是列出了本人认为经常在写存储过程中会遇到的一些具体的差别之处。