資料庫MySQL之如何新增主鍵、唯一、外來鍵和預設約束?
阿新 • • 發佈:2018-12-22
如何新增主鍵、唯一、外來鍵和預設約束?
文章目錄
1. 新增主鍵約束
CONSTRAINT_PK主鍵名字為PK
ALTER TABLE tbl_name ADD [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (index_col_name,...)
e.g:
ALTER TABLE users ADD CONSTRAINT PK_users_id PRIMARY KEY (id);
2. 新增唯一約束
ALTER TABLE tbl_name ADD [CONSTRAINT [symbol]] UNIQUE [INDEX|KEY] [index_name] [index_type] (index_col_name,...)
e.g:
ALTER TABLE users ADD UNIQUE (username);
3. 新增外來鍵約束
ALTER TABLE tbl_name ADD [CONSTRAINT [symbol]] FOREIGN KEY [index_name](index_col_name,...) reference_definition
e.g:
ALTER TABLE users ADD FOREIGN KEY (pid) REFERENCES provinces (id)
4. 新增/刪除預設約束 DEFAULT
ALTER TABLE tbl_name ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
e.g:
ALTER TABLE users ADD age TINYINT UNSIGNED NULL ;
ALTER TABLE users ALTER age SET DEFAULT 15;
ALTER TABLE users ALTER age DROP DEFAULT;