MySQL writes user-defined functions and stored procedure syntax (a detailed case is attached, and the problem has been solved: errors are reported when running user-defined functions, and errors are r

Problem scenario

Try to write mysql Function to sort the data in the record , When the first data is obtained according to the demand ,sql Medium limit @ Variable ,1 Syntax has never been compiled .
The error information is as follows ：

[Err] 1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '@N,1

Question why

limit n,m The syntax requirement of the statement is , The next two parameters must be integer constants , Cannot be a variable . So in the function select * from a order by xx limit @ Variable ,1 The function compilation fails .

resolvent

1、 Use integer constants

First, receive a dynamic integer parameter according to the function parameter , If the integer parameter does not need operation, it is directly placed in limit Back .
for example ： When select test(1); Will return to select xxx limit 1,1 The data of

CREATE FUNCTION test(N INT) RETURNS double
BEGIN
  RETURN select distinct gz from gzb order by gz desc limit N,1;
END

If the function parameter needs to be involved in the operation before it can get the actual value , Also use constants , because limit (n-1),1 It will also fail to compile . Because no matter 2-1 perhaps n-1 It's all variables . So we have to process the value in advance into a constant .
for example ：

CREATE FUNCTION test(N INT) RETURNS double
BEGIN
  set cl = N-1
  RETURN select distinct gz from gzb order by gz desc limit cl,1;
END

So that we don't make a mistake , because cl The value of has passed n-1 Only when you get it sql, and limit (n-1),1 perhaps set @cl = n-1;limit @cl,1 Wait until sql The value is calculated after execution , So it will fail to compile .

2、 Writing stored procedures + precompile

stay mysql-5.0.7 Later versions , Updated to preprocess sql sentence , When processing, put variables to execute .

CREATE PROCEDURE test(in n INT)
BEGIN
  set @mmsql = 'select distinct gz from gzb order by gz desc limit ?,1';
  set @cl = n-1;
  prepare exce from @mmsql;
  EXECUTE exce USING @cl;
  DEALLOCATE prepare exce;
END

Tips

Declare two types and ranges of variables

--  local variable 
--Declare  Variable name   type  default  The default value is ;
declare N int defaul 0;  --  Declare a variable named N Integral variable of , The default value is 0

--  Global variables 
-- set @ Variable name  =  value 
set @N = 'xxx';     --  Declare global variables @N, The value is a string xxx

Stored procedure syntax

Calling stored procedure ： call Store name ( The ginseng );
View all stored procedures ：show procedure status;
View the stored procedure creation statement ：show create procedure Store name ;
Delete stored procedure ：drop procedure Store name ;

create procedure  name (in|out|inout  Parameter name   Parameter type ) --  Multiple parameters are separated by commas 
begin
 -- sql sentence ;
end

-- in  The value representing the parameter is obtained from the user input ( The default is  in)
-- out  The value representing the parameter will be output to the user , But in stored procedures , The value passed in for this parameter is NULL
-- inout  It can receive parameters , You can also output parameters 

--  for example ：
set @a = 1;
set @b = 2;
set @c = 3;
select @a,@b,@c;  -- 1,2,3

create procedure demo(a int, out b int, inout c int)
begin
 select a,b,c;   -- 1,NULL,3
 set a = 10;
 set b = 10;
 set c = 10;
 select a,b,c;   -- 10,10,10
end

call demo(@a,@b,@c);

select @a,@b,@c;  -- 1,10,10

After the execution , Obviously @b,@c It's all changed

Function syntax

Call function ： select The name of the function ( The ginseng );
View all custom functions ：show function status;
View the function creation statement ：show create function The name of the function ;
Delete function ：drop function The name of the function ;

CREATE FUNCTION  The name of the function ( Parameter name   Parameter type ) RETURNS  return type 
begin
  -- xxxx
end

--  for example ： Output 1+2+...+n Result 
create function Accumulate(n int) returns int
begin
  declare i int default 1;
  declare sum int default 0;
  while i <= n do
    set sum = sum + i;
    set i = i + 1;
  end while;
  return sum;
end
--  Or use loop and leave Loop and jump out of the loop to achieve accumulation 
create function Accumulate(n int) RETURNS int
begin
  declare i int default 1;
  declare sum int default 0;
  add_num:LOOP
		set sum = sum + i;
    set i = i + 1;
    IF i = n + 1 THEN LEAVE add_num;
		END IF;
  END LOOP add_num;
  return sum;
end
--  Call function 
select Accumulate(5);  -- 15 = 1 + 2 + 3 + 4 + 5