MySQL之DATETIME与TIMESTAMP的时间精度问题

-- 创建数据库
CREATE DATABASE mydb_1;

-- 查看创建数据库建表语句（默认编码UTF8）
SHOW CREATE DATABASE mydb_1;

-- 创建表
-- 测试datetime的精度
CREATE TABLE test(
	-- 默认精度为0
	-- Maximum is 6.
	datetime1 DATETIME,
	datetime2 DATETIME(3),
	datetime3 DATETIME(5)
);
INSERT INTO test VALUES("2020-11-22 12:01:01.59999", "2020-11-22 12:01:01.59999", "2020-11-22 12:01:01.59999");
INSERT INTO test VALUES("2020-11-22 12:01:01.5", "2020-11-22 12:01:01.599", "2020-11-22 12:01:01.59999");
INSERT INTO test VALUES("2020-11-22 12:01:01.4", "2020-11-22 12:01:01.594", "2020-11-22 12:01:01.59994");
INSERT INTO test VALUES("2020-11-22 12:01:01.5", "2020-11-22 12:01:01.5995", "2020-11-22 12:01:01.599995");
INSERT INTO test VALUES("2020-11-22 12:01:01.5", "2020-11-22 12:01:01.5994", "2020-11-22 12:01:01.599994");


-- 创建表
-- 测试timestamp的精度
CREATE TABLE test11(
	-- 默认精度为0
	-- Maximum is 6.
	datetime1 TIMESTAMP,
	datetime2 TIMESTAMP(3),
	datetime3 TIMESTAMP(5)
);
INSERT INTO test1 VALUES("2020-11-22 12:01:01.59999", "2020-11-22 12:01:01.59999", "2020-11-22 12:01:01.59999");
INSERT INTO test1 VALUES("2020-11-22 12:01:01.5", "2020-11-22 12:01:01.599", "2020-11-22 12:01:01.59999");
INSERT INTO test1 VALUES("2020-11-22 12:01:01.4", "2020-11-22 12:01:01.594", "2020-11-22 12:01:01.59994");
INSERT INTO test1 VALUES("2020-11-22 12:01:01.5", "2020-11-22 12:01:01.5995", "2020-11-22 12:01:01.599995");
INSERT INTO test1 VALUES("2020-11-22 12:01:01.5", "2020-11-22 12:01:01.5994", "2020-11-22 12:01:01.599994");

-- 结束符修改为$$
DELIMITER $$
DROP PROCEDURE IF	EXISTS batch_alter_column_type $$
CREATE PROCEDURE batch_alter_column_type ( 
	sch_name VARCHAR ( 128 ), -- 库名称
	from_col_name VARCHAR ( 32 ), -- 修改的字段
	to_col_type VARCHAR ( 32 ) -- 修改之后的字段类型
) 
BEGIN
  -- 当前表名
	DECLARE tbl_name VARCHAR ( 64 );
  -- 当前字段名
	DECLARE col_name VARCHAR ( 64 );
	-- 游标结束标记
	DECLARE i_done INT ( 1 );
	-- 修改的SQL语句
	DECLARE SQL_FOR_ALTER VARCHAR ( 1024 );
	-- 声明游标，存储要修改表和字段
	DECLARE mycursor CURSOR FOR 
	   SELECT C.TABLE_NAME, C.COLUMN_NAME 
		 FROM INFORMATION_SCHEMA.COLUMNS C
		 LEFT JOIN INFORMATION_SCHEMA.TABLES T
		 ON C.TABLE_NAME = T.TABLE_NAME
		 AND C.TABLE_SCHEMA = T.TABLE_SCHEMA
	   WHERE C.TABLE_SCHEMA = sch_name  -- 字符串类型转换
	   AND TABLE_TYPE   = 'BASE TABLE'	 
		 AND COLUMN_NAME  = from_col_name;
	-- 游标中的内容执行完后将标记设置为1  
	DECLARE CONTINUE HANDLER FOR NOT FOUND SET i_done = 1;
  -- 打开游标	
	OPEN mycursor;
  -- 执行循环	
	Lp:LOOP	
		-- 取出游标中的值
		FETCH mycursor INTO tbl_name,col_name;
	  -- 如果标记为1，退出循环
		IF i_done = 1 THEN 
			 LEAVE Lp;
		END IF;
		-- 构造修改语句
		SET SQL_FOR_ALTER = CONCAT( "ALTER TABLE ", tbl_name, " MODIFY COLUMN ", col_name, " ", to_col_type );
		-- 给局部变量赋值
		SET @SQL = SQL_FOR_ALTER;
		-- 预处理SQL语句
		PREPARE stmt FROM @SQL;
		-- 执行SQL语句
		EXECUTE stmt;
	END LOOP;
	-- 释放游标
	CLOSE mycursor;
END
$$
-- 调用存储过程
DELIMITER ;
call batch_alter_column_type('mydb_1','MODISTAMP', 'datetime(3)');

-- 修改test表中datetime1字段属性为DATETIME(3)
ALTER TABLE test MODIFY COLUMN datetime1 DATETIME(3);

-- 修改test表中datetime1字段名称为datetime11，属性为DATETIME(2)
ALTER TABLE test CHANGE datetime1 datetime11 DATETIME(2);