MySQL 如何在MySQL数据库中创建复合键
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how to create a composite key in MySQL database
提问by Ritabrata Gautam
i am working on mysql server.where i have created a table, named question . column/attributes of this table are (course,subject,year,question) i want to create a primary key(or composite key) consists of (course+subject+year). i.e. for a particular course+subject+year combination there can be only one question.there will be only one row with the combination of (course+subject+year),creation of another row won't be possible. i have done it by :
我在 mysql server.where 上工作，我创建了一个名为 question 的表。该表的列/属性是（课程、学科、年份、问题）我想创建一个由（课程+学科+年份）组成的主键（或复合键）。即对于特定的课程+学科+年份组合，只能有一个问题。只有一行是（课程+学科+年份）的组合，无法创建另一行。我已经做到了：
primary key(course,subject,year);
but it's not working.still i can create two rows with same combination of course,subject,year.
但它不起作用。我仍然可以用相同的课程、主题、年份组合创建两行。
can anyone tell me how can i create a composite key propery????
谁能告诉我如何创建复合键属性？？？？
回答by Ritabrata Gautam
the syntax is CONSTRAINT constraint_name PRIMARY KEY(col1,col2,col3)for example ::
语法是CONSTRAINT constraint_name PRIMARY KEY(col1,col2,col3)例如 ::
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
the above example will work if you are writting it while you are creating the table for example ::
如果您在创建表格时编写它，则上面的示例将起作用，例如 ::
CREATE TABLE person (
P_Id int ,
............,
............,
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
);
to add this constraint to an existing table you need to follow the following syntax
将此约束添加到现有表中，您需要遵循以下语法
ALTER TABLE table_name ADD CONSTRAINT constraint_name PRIMARY KEY (P_Id,LastName)
回答by bizzehdee
if it is mysql you are looking at, you should do something similar to
如果您正在查看的是 mysql，则应该执行类似于
ALTER TABLE table_name ADD PRIMARY KEY (a, b, c);
