DB2 修改列属性 设置默认值

 1.更改类型(设置为主键的列不能更改类型)
 ALTER TABLE "SCHEMA"."TABLENAME" ALTER COLUMN "COL" SET DATA TYPE VARCHAR(32);
 2.更改默认值
 ALTER TABLE "SCHEMA"."TABLENAME" ALTER COLUMN "COL" SET DEFAULT 'ABC';
 系统默认值：
 ALTER TABLE "SCHEMA"."TABLENAME" ALTER COLUMN "COL" SET DEFAULT; --设置默认值
 ALTER TABLE "SCHEMA"."TABLENAME" ALTER COLUMN "COL" DROP DEFAULT; --删除默认值
 3.更改是否允许空值
 ALTER TABLE "SCHEMA"."TABLENAME" ALTER COLUMN "COL" SET NOT NULL;
 ALTER TABLE "SCHEMA"."TABLENAME" ALTER COLUMN "COL" DROP NOT NULL;
 更改列类型，是否允许空值后，需要执行REORG TABLE "TABLENAME";
 更改默认值后，通常需要执行UPDATE "SCHEMA"."TABLENAME" SET "COL" = DEFAULT WHERE "COL" IS NULL;