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How to determine the uniqueness of a constraint in MySQL?
MySQL uses the combination of values in both column column_name1 and column_name2 to evaluate the uniqueness. If you define a UNIQUE constraint without specifying a name, MySQL automatically generates a name for it. To define a UNIQUE constraint with a name, you use this syntax: [CONSTRAINT constraint_name] UNIQUE (column_list)
Can a FOREIGN KEY constraint be defined in MySQL 5.7?
In MySQL 5.7.13 and earlier, InnoDB does not permit defining a foreign key constraint with a cascading referential action on the base column of an indexed virtual generated column. This restriction is lifted in MySQL 5.7.14.
Which is an example of a unique constraint?
For example, email addresses of users in the users table, or phone numbers of customers in the customers table should be unique. To enforce this rule, you use a UNIQUE constraint. A UNIQUE constraint is an integrity constraint that ensures values in a column or group of columns to be unique.
How to create a FOREIGN KEY constraint for a parent table?
Parent and child tables must use the same storage engine, and they cannot be defined as temporary tables. Creation of a foreign key constraint requires the REFERENCES privilege for the parent table. Corresponding columns in the foreign key and the referenced key must have similar data types.
Can a unique constraint contain a null value?
None of the fields that are part of the primary key can contain a null value. Some of the fields that are part of the unique constraint can contain null values as long as the combination of values is unique. The syntax for creating a unique constraint using a CREATE TABLE statement in MySQL is:
What is the difference between a unique constraint and a primary key?
A unique constraint is a single field or combination of fields that uniquely defines a record. Some of the fields can contain null values as long as the combination of values is unique. What is the difference between a unique constraint and a primary key?
What are the unique indexes in mysql table?
As you can see from the output, MySQL created two UNIQUE indexes on the suppliers table: phone and uc_name_address. The following SHOW INDEX statement displays all indexes associated with the suppliers table.
How to name unique constraint on multiple columns?
To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax: ALTER TABLE Persons ADD CONSTRAINT UC_Person UNIQUE (ID,LastName);
Can a multi column unique index work in MySQL?
Multi column unique indexes do not work in MySQL if you have a NULL value in row as MySQL treats NULL as a unique value and at least currently has no logic to work around it in multi-column indexes.