DB2 設置列的類型、缺省值（默認值）、非空等

1.更改列的類型(設置主鍵的列不能更改類型)
ALTER TABLE "SCHEMA"."TABLENAME" ALTER COLUMN "COL" SET DATA TYPE VARCHAR(32);

2.更改/設置缺省值（默認值） 
ALTER TABLE "SCHEMA"."TABLENAME" ALTER COLUMN "COL" SET DEFAULT 'ABC';

系統默認值：
ALTER TABLE "SCHEMA"."TABLENAME" ALTER COLUMN "COL" SET DEFAULT; --設置默認值
ALTER TABLE "SCHEMA"."TABLENAME" ALTER COLUMN "COL" DROP DEFAULT; --刪除默認值

3.更改是否允許空值
ALTER TABLE "SCHEMA"."TABLENAME" ALTER COLUMN "COL" SET NOT NULL;
ALTER TABLE "SCHEMA"."TABLENAME" ALTER COLUMN "COL" DROP NOT NULL;

更改列類型，是否允許空值后，需要執行REORG TABLE "TABLENAME";
更改默認值后，通常需要執行UPDATE "SCHEMA"."TABLENAME" SET "COL" = DEFAULT WHERE "COL" IS NULL;