CREATE TABLE `null_index_t` (
 `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
 `null_key` varchar(255) DEFAULT NULL,
 `null_key1` varchar(255) DEFAULT NULL,
 `null_key2` varchar(255) DEFAULT NULL,
 PRIMARY KEY (`id`),
 KEY `idx_1` (`null_key`) USING BTREE,
 KEY `idx_2` (`null_key1`) USING BTREE,
 KEY `idx_3` (`null_key2`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

delimiter $	#以delimiter来标记用$表示存储过程结束
create procedure nullIndex1()
begin
declare i int;	
declare j int;	
set i=1;
set j=1;
while(i<=100) do	
	while(j<=100) do	
		IF (i % 3 = 0) THEN
	   INSERT INTO null_index_t ( `null_key`, `null_key1`, `null_key2` ) VALUES (null , LEFT(MD5(RAND()), 8), LEFT(MD5(RAND()), 8));
  ELSEIF (i % 3 = 1) THEN
			 INSERT INTO null_index_t ( `null_key`, `null_key1`, `null_key2` ) VALUES (LEFT(MD5(RAND()), 8), NULL, LEFT(MD5(RAND()), 8));
	 ELSE
			 INSERT INTO null_index_t ( `null_key`, `null_key1`, `null_key2` ) VALUES (LEFT(MD5(RAND()), 8), LEFT(MD5(RAND()), 8), NULL);
  END IF;
		set j=j+1;
	end while;
	set i=i+1;
	set j=1;	
end while;
end 
$
call nullIndex1();

EXPLAIN select * from null_index_t WHERE null_key is null;

EXPLAIN select * from null_index_t WHERE null_key is not null;

EXPLAIN select * from null_index_t WHERE null_key is null;

EXPLAIN select * from null_index_t WHERE null_key is not null;