DROP PROCEDURE IF EXISTS TRYADDTABLE;

delimiter //
CREATE PROCEDURE TRYADDTABLE (IN tableName VARCHAR(20),IN createTableSql VARCHAR(1000))
BEGIN
	DECLARE tableCount INT;
  SELECT COUNT(*) INTO tableCount FROM information_schema.TABLES WHERE TABLE_NAME = tableName;	
	IF(tableCount = 0) THEN
	  -- @表示全局变量 相当于php $ 拼接赋值 INTO 必须要用全局变量不然语句会报错
		SET @create_sql = createTableSql;
		-- 预处理需要执行的动态SQL，其中stmt是一个变量
		PREPARE stmt FROM @create_sql;
		-- 执行SQL语句
    EXECUTE stmt;
		-- 释放掉预处理段
		deallocate prepare stmt;
	END IF;
END //
delimiter ;

CALL TRYADDTABLE('person','CREATE TABLE person (
person_id INT NOT NULL PRIMARY KEY,
fname VARCHAR(40) NULL,
lname VARCHAR(40) NULL,
created TIMESTAMP
);');

SELECT * FROM person;

DROP PROCEDURE IF EXISTS TRYADDTABCOLUMN;

delimiter //
CREATE PROCEDURE TRYADDTABCOLUMN (IN tableName VARCHAR(50),IN colName VARCHAR(20),IN colType VARCHAR(20))
BEGIN
	DECLARE colCount INT;
  SELECT COUNT(*) INTO colCount FROM information_schema.COLUMNS WHERE TABLE_NAME = tableName AND COLUMN_NAME = colName;	
	IF(colCount = 1) THEN
	  -- @表示全局变量 相当于php $ 拼接赋值 INTO 必须要用全局变量不然语句会报错
		SET @drop_sql = CONCAT('ALTER TABLE ',tableName,' DROP COLUMN ',colName,';');
		-- 预处理需要执行的动态SQL，其中stmt是一个变量
		PREPARE stmt FROM @drop_sql;
		-- 执行SQL语句
    EXECUTE stmt;
		-- 释放掉预处理段
		deallocate prepare stmt;	
	END IF;
		-- @表示全局变量 相当于php $ 拼接赋值 INTO 必须要用全局变量不然语句会报错
	SET @add_sql = CONCAT('ALTER TABLE ',tableName,' ADD COLUMN ',colName,' ',colType,';');
	-- 预处理需要执行的动态SQL，其中stmt是一个变量
	PREPARE stmt FROM @add_sql;
	-- 执行SQL语句
	EXECUTE stmt;
	-- 释放掉预处理段
	deallocate prepare stmt;	
END //
delimiter ;

CALL TRYADDTABCOLUMN('person','fname','VARCHAR(20)');
CALL TRYADDTABCOLUMN('person','updated','TIMESTAMP');

SHOW CREATE TABLE person;