SQL statement-exec execution

Dynamic SQL statement Basic syntax
1: Normal SQL statements can be executed with exec

Eg:select * Fromtablename
Exec (' select * from TableName ')
Exec sp_executesql n ' select * fromtablename '--please note that the string must be added n before

2: Field name, table name, database name, etc. as variables, you must use dynamic SQL

eg
DECLARE @fname varchar (20)
Set @fname = ' Filedname '
Select @fname Fromtablename-Error, does not prompt for errors, but results are fixed values filedname, not required.
Exec (' select ' + @fname + ' fromtablename ')--note the plus space on the edge of the single quotation mark before and after

Of course, changing the string to a variable can also
DECLARE @fname varchar (20)
Set @fname = ' Filedname '--set field name

DECLARE @s varchar (1000)
Set @s = ' SELECT ' + @fname + ' from TableName '
Exec (@s)--Success
EXEC sp_executesql @s--This sentence will be an error

DECLARE @s Nvarchar (1000)--note here to Nvarchar (1000)
Set @s = ' SELECT ' + @fname + ' from TableName '
Exec (@s)--Success
EXEC sp_executesql @s--this sentence is correct

3. Output parameters
DECLARE @num int,
@sqls nvarchar (4000)
Set @sqls = ' SELECT count (*) from TableName '
EXEC (@sqls)
--How do I put the exec execution result into a variable?

DECLARE @num int,
@sqls nvarchar (4000)
Set @sqls = ' Select @a=count (*) from TableName '
EXEC sp_executesql @sqls, N ' @a int output ', @num output
Select @num

SQL statement-exec execution