外键是用于链接两个表的键。具有外键约束（又名“子表”）的表连接到另一个表（也称为“父表”）。连接位于子表的外键约束和父表的主键之间。
外键约束用于帮助维护表之间的一致性。例如，如果删除父表记录并且子表具有记录，则系统也可以删除子记录。
它们还通过要求在子表中输入的每个记录存在父表记录来帮助防止在子表中输入不准确的数据。
使用示例
对于本指南，我们将仔细查看学生（家长）和学生联系（孩子）表。
父表的主键
请注意，student表具有studentID的一列主键。
SHOW index FROM student;
+---------+------------+----------+--------------+-------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name |
+---------+------------+----------+--------------+-------------+
| student | 0 | PRIMARY | 1 | studentID |
+---------+------------+----------+--------------+-------------+
1 row in set (0.00 sec) (some columns removed on the right for clarity)
子表的主键和外键
学生联系信息表有一个主键，也是studentID。这是因为两个表之间存在一对一的关系。换句话说，我们希望每个学生只有一个学生和一个学生联系记录。
SHOW index FROM `student-contact-info`;
+----------------------+------------+----------+--------------+-------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name |
+----------------------+------------+----------+--------------+-------------+
| student-contact-info | 0 | PRIMARY | 1 | studentID |
+----------------------+------------+----------+--------------+-------------+
1 row in set (0.00 sec) (some columns removed on the right for clarity)
SELECT concat(table_name, '.', column_name) AS 'foreign key',
concat(referenced_table_name, '.', referenced_column_name) AS 'references'
FROM information_schema.key_column_usage
WHERE referenced_table_name IS NOT NULL
AND table_schema = 'fcc_sql_guides_database'
AND table_name = 'student-contact-info';
+--------------------------------+-------------------+
| foreign key | references |
+--------------------------------+-------------------+
| student-contact-info.studentID | student.studentID |
+--------------------------------+-------------------+
1 row in set (0.00 sec)
使用学生父表和联系子表的示例报告
SELECT a.studentID, a.FullName, a.programOfStudy,
b.`student-phone-cell`, b.`student-US-zipcode`
FROM student AS a
JOIN `student-contact-info` AS b ON a.studentID = b.studentID;
+-----------+------------------------+------------------+--------------------+--------------------+
| studentID | FullName | programOfStudy | student-phone-cell | student-US-zipcode |
+-----------+------------------------+------------------+--------------------+--------------------+
| 1 | Monique Davis | Literature | 555-555-5551 | 97111 |
| 2 | Teri Gutierrez | Programming | 555-555-5552 | 97112 |
| 3 | Spencer Pautier | Programming | 555-555-5553 | 97113 |
| 4 | Louis Ramsey | Programming | 555-555-5554 | 97114 |
| 5 | Alvin Greene | Programming | 555-555-5555 | 97115 |
| 6 | Sophie Freeman | Programming | 555-555-5556 | 97116 |
| 7 | Edgar Frank "Ted" Codd | Computer Science | 555-555-5557 | 97117 |
| 8 | Donald D. Chamberlin | Computer Science | 555-555-5558 | 97118 |
+-----------+------------------------+------------------+--------------------+--------------------+
结论
外键约束是一种很好的数据完整性工具。花点时间学习它们。
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