mysql foreign key is a very useful data processing function that can quickly update data between tables; simply speaking, it can establish an association between two tables and perform operations When one table is updated, the data in the other table will also change simultaneously.
The operating environment of this tutorial: Windows 10 system, mysql8 version, Dell G3 computer.
What is mysql foreign key?
##Usage of Mysql foreign keys
Foreign key is a very, very easy-to-use thing, and it is also a function that many relational databases have. To put it simply, it can establish an association between two tables, which can be done when operating one table. , the data in another table will also change simultaneously.
Example: 一個(gè)學(xué)生表里面有學(xué)生的所有信息,其中有一個(gè)字段是班級(jí)id,又有一個(gè)班級(jí)表,記錄著所有的班級(jí)信息,按照邏輯來說,如果我們刪除了班級(jí)
表里面的某個(gè)班級(jí),學(xué)生表里面是這個(gè)班級(jí)id的信息就應(yīng)該修改。
The above example can be completed normally if we use a program. First delete the class, and then add it to the student table. The class id field of the student information of this class is modified and divided into two parts. But if we use foreign keys, it only takes one step. As long as one of the foreign keys is associated, the other one will be automatically updated. Obviously, this is more in line with the program. It is also easier
Details before use
- When selecting a storage engine for the database, you must choose a storage engine that adapts to foreign keys, such as the default storage of mysql Engine innodb
- The field types of related foreign key fields must be consistent. For example, the class_id of the student table is of type int, then the type of class_id of the class table should also be of type int. This is very important.
Create foreign key constraints in the new table create?table?stu_study?(
??sid?int?primary?key?auto_increment,--?定義Sid??
??????sname?varchar(15)?not?null,
??????course_id?int?default?null,??--?定義課程id?課程id是外鍵所以要與關(guān)聯(lián)的主表的字段類型保持一致
??????constraint?stu_study_class?--?一個(gè)表里可能有多個(gè)表之間關(guān)聯(lián),所以外鍵需要起一個(gè)名字
??????foreign?key?(class_id)?--?關(guān)聯(lián)的外鍵名
??????references?classes(id)??--?關(guān)聯(lián)的主表和主表的字段
??????on?delete?cascade??--?當(dāng)刪除的時(shí)候觸發(fā)
????)engine=InnoDB?default?charset?utf8??--?默認(rèn)存儲(chǔ)引擎和編碼的字符串
In the existing table Add and delete foreign keys in
- Delete foreign keys
- ···
– Delete foreign keys (foreign key constraints)
alter table stu_study drop foreign key stu_study_classes;
···Deleting a foreign key is done by deleting it based on the foreign key name in the table. This also indirectly lets us know that the foreign key name cannot be repeated.
Add foreign key -
--?添加外鍵 ??alter?table?stu_study?add ??constraint?stu_study_classes??--?外鍵名?是一定不能夠重復(fù)的,通常會(huì)用關(guān)聯(lián)的兩個(gè)表名進(jìn)行命名 ??foreign?key(course_id) ??references?classes(id) ??on?delete?cascade;
Adding a foreign key to an existing table is very similar to adding a foreign key when creating a table. There are some details. That is, he needs to use the constraint foreign key name to set the name of the foreign key
We will find that adding and modifying foreign keys is very similar to adding and modifying table fields. The keywords used are add, drop, etc.
Operation of associated actions
Sub table Main table
We can simply understand the word table and sub table as , the table with the foreign key set is the subtable
There are many kinds of related actions, such as
cascade,
set Null,
no action, all three It is to set the associated action when setting the foreign key. For example,
represents the action performed when deleting. The main differences are as follows:
-
cascade
刪除主表的某個(gè)字段的時(shí)候,子表含有這個(gè)字段的數(shù)據(jù)會(huì)被清空,這個(gè)還是屬于相對(duì)危險(xiǎn)的一個(gè)操作的 -
+set null
刪除主表的某個(gè)字段的時(shí)候,子表含有這個(gè)字段的這個(gè)哪一行的這個(gè)字段會(huì)用null來顯示,但是有個(gè)細(xì)節(jié)就是設(shè)置外鍵的這個(gè)字段字段類型就不能設(shè)置為not null類型的,否則會(huì)報(bào)錯(cuò) -
no action
這個(gè)字段會(huì)比較有意思,也就是說當(dāng)主表要?jiǎng)h除某個(gè)行的時(shí)候,如果外鍵關(guān)聯(lián)有含有這個(gè)主表的外鍵的字段數(shù)據(jù)的話,就不會(huì)刪除成功,系統(tǒng)會(huì)直接報(bào)錯(cuò)
關(guān)聯(lián)更新操作
之前是主要講了關(guān)聯(lián)刪除,是因?yàn)橥怄I在使用的時(shí)候關(guān)聯(lián)刪除操作是使用的比較頻繁的,關(guān)聯(lián)更新的頻率是相對(duì)來說低一點(diǎn)兒的
關(guān)聯(lián)更新的三個(gè)關(guān)聯(lián)操作和刪除時(shí)一樣的,分別是cascade
,set null
,no action
意思是也是一樣的,主表某個(gè)字段更新了,子表也會(huì)更新那個(gè)字段!!!,主表更新的某條數(shù)據(jù),子表的使用的那個(gè)數(shù)據(jù)會(huì)變成空,和子表在使用的情況下,主表就不能夠更新數(shù)據(jù)
alter?table?stu_study?drop?foreign?key?stu_study_classes;??--?刪除外鍵 --?添加外鍵 alter?table?stu_study?add? ????constraint?stu_study_classes? ????foreign?key?(course_id)? ????references?classes(id)? ????on?delete?set?null?--?一次性設(shè)置外鍵的兩種動(dòng)作 ????on?update?cascade
總結(jié)
- 數(shù)據(jù)庫的外鍵是非常非常好用的一個(gè)技術(shù),可以讓我們快速的進(jìn)行表之間的數(shù)據(jù)的更新
- 外鍵可以簡(jiǎn)單的理解成會(huì)自動(dòng)的替我們做一個(gè)數(shù)據(jù)變動(dòng)的處理
推薦學(xué)習(xí):《MySQL視頻教程》
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