【DATABASE-Mysql(3)】删除/添加字段，判断字段是否存在

1、删除字段

1.1、不作字段是否存在的判断

ALTER TABLE `ssdds2`.`medicine` DROP COLUMN `med_price` ;

1.2、判断字段是否存在

SET @sch = 'ssdds2';SET @tab = 'medicine';SET @col = 'med_price';
delimiter //
CREATE PROCEDURE tmp_drop_column(IN S VARCHAR(512),IN T VARCHAR(512),IN C VARCHAR(512)) BEGIN
  DECLARE tC varchar(200);
	DECLARE tS varchar(200);
  SET tC = NULL;
  SELECT column_name INTO tC FROM information_schema.`COLUMNS` WHERE  table_schema = S AND table_name = T AND column_name = C;
  IF !ISNULL(tC) THEN
	  SET tS = `CONCAT`('ALTER TABLE `',S,'`.`',T,'` DROP COLUMN ',C);
	  SET @t = tS;
	  PREPARE tE from @t;
	  EXECUTE tE;
	  DEALLOCATE PREPARE tE;
  END IF;
END;
//
delimiter ';'

CALL tmp_drop_column(@sch,@tab,@col);
DROP PROCEDURE tmp_drop_column;

2、添加字段

2.1、不作字段是否存在的判断

ALTER TABLE `ssdds`.`medicine` ADD COLUMN `med_price`  varchar(30) CHARACTER SET gbk COLLATE gbk_chinese_ci NULL DEFAULT NULL COMMENT '药品价格' AFTER `url_picture`;

2.2、判断字段是否存在

SET @sch = 'ssdds2';SET @tab = 'medicine';SET @col = 'med_price';
SET @par = "varchar(30) CHARACTER SET gbk COLLATE gbk_chinese_ci NULL DEFAULT NULL COMMENT '药品价格' AFTER `url_picture`;";
delimiter //
CREATE PROCEDURE tmp_add_column(IN S VARCHAR(512),IN T VARCHAR(512),IN C VARCHAR(512),IN P VARCHAR(512)) BEGIN
  DECLARE tC varchar(200);
	DECLARE tS varchar(200);
  SET tC = NULL;
  SELECT column_name INTO tC FROM information_schema.`COLUMNS` WHERE  table_schema = S AND table_name = T AND column_name = C;
  IF ISNULL(tC) THEN
	  SET tS = `CONCAT`('ALTER TABLE `',S,'`.`',T,'` ADD COLUMN ',C,' ',P);
	  SET @t = tS;
	  PREPARE tE from @t;
	  EXECUTE tE;
	  DEALLOCATE PREPARE tE;
  END IF;
END;
//
delimiter ';'

CALL tmp_add_column(@sch,@tab,@col,@par);
DROP PROCEDURE tmp_add_column;