ALTER TABLE provider ADD PRIMARY KEY(person,place,thing);

如果已经存在主要关键,那么你就希望这样做。

ALTER TABLE provider DROP PRIMARY KEY, ADD PRIMARY KEY(person, place, thing);

@Adrian Cornish的回答是正确的。 然而,还有另一个告诫要放弃现有的主要钥匙。 如果这一主要钥匙被另一张桌子用作外国钥匙,那么你在试图放弃该钥匙时会遇到错误。 在一些版本的神秘电文中,错误电文被误报(截至5.5.17,这一错误电文仍然存在。

alter table parent  drop column id;
ERROR 1025 (HY000): Error on rename of
 ./test/#sql-a04_b  to  ./test/parent  (errno: 150).

如果你想要放弃另一个表格提到的主要钥匙,你将不得不在另一个表格中首先放弃外国钥匙。 如果你在重新确定主要钥匙后仍然想要外国钥匙,你可以重新研究。

Also, when using composite keys, order is important. These

1) ALTER TABLE provider ADD PRIMARY KEY(person,place,thing);
and
2) ALTER TABLE provider ADD PRIMARY KEY(person,thing,place);

are not the the same thing. They both enforce uniqueness on that set of three fields, however from an indexing standpoint there is a difference. The fields are indexed from left to right. For example, consider the following queries:

A) SELECT person, place, thing FROM provider WHERE person =  foo  AND thing =  bar ;
B) SELECT person, place, thing FROM provider WHERE person =  foo  AND place =  baz ;
C) SELECT person, place, thing FROM provider WHERE person =  foo  AND place =  baz  AND thing =  bar ;
D) SELECT person, place, thing FROM provider WHERE place =  baz  AND thing =  bar ;

B can use the primary key index in ALTER statement 1
A can use the primary key index in ALTER statement 2
C can use either index
D can t use either index

A将指数2的前两个领域作为部分指数。 a 不能使用指数1,因为它不知道指数的中间位置部分。 即便如此,它仍可能只使用部分指数。

D可以不使用指数,因为它没有人知道。

See the mysql docs here, for more information.

You may simply want a UNIQUE CONSTRAINT. Especially if you already have a surrogate key. (example of an already existing surrogate key would be a single column that is an AUTO_INCREMENT )

下面是《联邦制约法》的缩略语。

ALTER TABLE `MyDatabase`.`Provider`
    ADD CONSTRAINT CK_Per_Place_Thing_Unique UNIQUE (person,place,thing)
;

alter table table_name add primary key (col_name1, col_name2);

www.un.org/Depts/DGACM/index_arabic.htm

如@GranadaCoder提供的,使用COMOSITE UNI Request KEY肯定会更好。

<编码> ALTER IGNORE TABLE table_name ADD UNI RequestS INDEX idx_name(about_id, another_id, one_more_id);