How do I code a foreign key in MySQL?
MySQL defines the foreign key in two ways: Using CREATE TABLE Statement.
Define Foreign Key Using ALTER TABLE Statement
- ALTER TABLE table_name.
- ADD [CONSTRAINT [symbol]] FOREIGN KEY.
- [index_name] (column_name, …)
- REFERENCES table_name (column_name,…)
- ON DELETE referenceOption.
- ON UPDATE referenceOption.
Can any column be a foreign key?
Yes , There can be a foreign key which is unique key in other table as Unique key is subset of primary key but not the exact primary key.
How do I add a column to a foreign key in a table?
Add new column with foreign key constraint in one command
- ALTER TABLE one.
- ADD two_id integer;
- ALTER TABLE one.
- ADD FOREIGN KEY (two_id) REFERENCES two(id);
Can foreign key be NULL?
Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table). That is all an FK is by definition.
Can foreign keys be NULL MySQL?
5 Answers. NULLs in foreign keys are perfectly acceptable. Dealing with NULLs in foreign keys is tricky but that does not mean that you change such columns to NOT NULL and insert dummy (“N/A”, “Unknown”, “No Value” etc) records in your reference tables.
Can a foreign key be non primary key?
Yes, a foreign key can reference a non-primary key which is unique. A FOREIGN KEY constraint does not have to be linked only to a PRIMARY KEY constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table.
Can I use a non primary key as a foreign key?
Yes. You can FK reference any UNIQUE KEY constraint (one or more columns).
Is foreign key should be primary key?
A foreign key is a reference to a unique identifier. Theoretically the reference should be to a primary key and not some other unique key. … Unfortunately, in practice many databases have foreign key problems – about 50% of databases, we’ve found. Here are some common foreign key problems.
What is a foreign key column?
A foreign key is a column (or combination of columns) in a table whose values must match values of a column in some other table. FOREIGN KEY constraints enforce referential integrity, which essentially says that if column value A refers to column value B, then column value B must exist.
What is a foreign key example?
A foreign key is a set of attributes in a table that refers to the primary key of another table. The foreign key links these two tables. … For example, a table called TEAM may have an attribute, MEMBER_NAME, which is a foreign key referencing a candidate key, PERSON_NAME, in the PERSON table.