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如何添加主键、唯一、外键和默认约束?
1. 添加主键约束
CONSTRAINT_PK主键名字为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. 添加唯一约束
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. 添加外键约束
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. 添加/删除默认约束 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;