为什么删除了主键primary之后唯一约束会变成主键约束？还有为什么删除不了？必须要有约束们
Why does the unique constraint become a primary key constraint after deleting the primary key? -PHP Chinese website Q&A-Why does the unique constraint become a primary key constraint after deleting the primary key? -PHP Chinese website Q&A
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主键约束和唯一约束都具有唯一标识数据表记录的作用，数据表记录千千万万，必然存在标志唯一性的字段，主键约束存在时，主键起标识唯一性作用，主键删除后，从一个或多个唯一约束中顺序选择其中一个作为唯一标识，
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Why does the unique constraint become a primary key constraint after deleting the primary key? -PHP Chinese website Q&A-Why does the unique constraint become a primary key constraint after deleting the primary key? -PHP Chinese website Q&A
Please watch and learn.
主键约束和唯一约束都具有唯一标识数据表记录的作用，数据表记录千千万万，必然存在标志唯一性的字段，主键约束存在时，主键起标识唯一性作用，主键删除后，从一个或多个唯一约束中顺序选择其中一个作为唯一标识，