sqlserver、达梦、mysql的差异

差异项	sqlserver	达梦	mysql
单行注释	--	--	1、-- ，--后面带个空格 2、#
包裹对象名称，如表、表字段等	[tableName]	"tableName"	`tableName`
表字段自增	IDENTITY(1, 1)	IDENTITY(1, 1)	AUTO_INCREMENT
二进制数据类型	IMAGE	IMAGE、BLOB	BLOB
存储一个汉字需要定义的varchar长度	varchar(2)	varchar(3)	varchar(1)
判断是否为NUL	isnull(id, 0)	isnull(id, 0)	ifnull(id, 0)
获取当前日期/时间	select getdate()	1、select getdate(); 2、select now();	SELECT NOW();
类型转换convert，将日期转换为字符串，返回yyyy-mm-dd	select CONVERT(varchar(10), getdate(), 121)	1、select SUBSTR(CONVERT(varchar(30), getdate()), 1, 10) 2、select SUBSTR(CONVERT(varchar(30), now()), 1, 10)	SELECT CONVERT(NOW(), CHAR(10))
判断字符串中是否包含某些字符	charindex('字符', '全部字符串')	1、instr('全部字符串', '字符') 2、regexp_like('全部字符串', '字符')	INSTR('全部字符串', '字符')
字符串拼接操作符	+ select 's1' + 's2' + 's3'	\|\| SELECT 's1' \|\| 's2' \|\| 's3';	CONCAT SELECT CONCAT('s1', 's2', 's3');
求字符串长度	select len('abc')	1、select len('abc')； 2、select length('abc');	SELECT LENGTH('abc');
执行语句块，变量定义、赋值、运算、查询等	declare @i int, @j int, @k int set @i = 1 set @j = 2 set @k = @i + @j select @k as k	DECLARE i,j,k INT; begin set i = 1; set j = 2; set k = i + j; select k as k; end;	SET @i = 1; SET @j = 2; SET @k = @i + @j; SELECT @k AS k;
执行/调用存储过程	exec proc_GetMenu 'manager'	1、proc_GetMenu('manager'); 2、call proc_GetMenu('manager'); 3、exec proc_GetMenu 'manager'; 4、execute proc_GetMenu 'manager';	CALL proc_GetMenu('manager');
指定查询返回一条数据	select top 1 * from employee	select top 1 * from employee	SELECT * FROM employee LIMIT 1
delete	1、delete Employee where emp_ID = 'userId' 2、delete from Employee where emp_ID = 'userId'	1、delete Employee where emp_ID = 'userId'; 2、delete from Employee where emp_ID = 'userId';	DELETE FROM Employee WHERE emp_ID = 'userId';
if…else语句	declare @age int; set @age = 18; if @age < 18 begin select '小于18' AS msg; end else begin select '已满18' as msg; end;	declare age int; begin set age = 18; if age < 18 begin select '小于18' AS msg; end else begin select '已满18' as msg; end; end;	不能用于会话，只能用于存储过程、存储函数、触发器中。
临时表	create table #tempTable(id int, xm varchar(20)) 表名必须以#开头	CREATE GLOBAL TEMPORARY TABLE tmpTable(id int, xm varchar(30));	CREATE TEMPORARY TABLE tmpTable(id INT, xm VARCHAR(10));
建表主键的关键语句	CONSTRAINT [PK_name] PRIMARY KEY CLUSTERED ( [ID] ASC )	NOT CLUSTER PRIMARY KEY("ID")	PRIMARY KEY (`id`)