In the created table, a primary key is placed first, followed by all unique indexes, and then the nonunique indexes. this helps the mysql optimizer to prioritize which index to use and also more quickly to detect duplicated unique keys. a primary key can be a multiple-column index.. Adding a composite unique key to an existing mysql table tutorial explaining how to add a composite unique key to an existing mysql table, after firstly checking for existing violations.. This section discusses the relationship of partitioning keys with primary keys and unique keys. the rule governing this relationship can be expressed as follows: all columns used in the partitioning expression for a partitioned table must be part of every unique key that the table may have..
Using hash prevents the creation of an ordered index; otherwise, creating a unique index or primary key on an ndb table automatically results in the creation of both an ordered index and a hash index, each of which indexes the same set of columns.. Primary key vs. unique key vs. key. key is the synonym for index.you use the key when you want to create an index for a column or a set of columns that is not the part of a primary key or unique key. a unique index creates a constraint for a column whose values must be unique. unlike the primary index, mysql allows null values in the unique index. in addition, a table can have multiple unique. Introduction to mysql unique constraint. sometimes, you want to enforce the uniqueness value in a column e.g., the phones of the suppliers in the suppliers table must be unique, or the combination of the supplier name and address must not be duplicate. to enforce this rule, you need to use the unique constraint..
