SQL 基础知识梳理：主键（PK）和外键（FK）

CREATE TABLE table_test(


id INT NOT NULL,


name VARCHAR(20) NOT NULL,


address VARCHAR(20),


PRIMARY KEY(id)


);
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CREATE TABLE table_test(


id INT NOT NULL,


name VARCHAR(20) NOT NULL,


address VARCHAR(20)


);


ALTER TABLE table_test ADD PRIMARY KEY (id);
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CREATE TABLE table_test(


user_id INT NOT NULL,


user_name VARCHAR(20) NOT NULL,


user_address VARCHAR(20),


PRIMARY KEY (user_id, user_name)


);
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CREATE TABLE table_test(


user_id INT NOT NULL,


user_name VARCHAR(20) NOT NULL,


user_address VARCHAR(20)


);


ALTER TABLE table_test ADD CONSTRAINT id PRIMARY 
KEY (user_id, user_name);
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ALTER TABLE table_test DROP PRIMARY KEY ;
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