UNIQUE (ID) ); To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:CREATE TABLE Persons ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, CONSTRAINT UC_Person UNIQUE (ID,LastName) ); UNIQUE...
If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within the prefix length. A UNIQUE index permits multiple NULL values for columns that can contain NULL. 官方的文档中明确说明在唯一索引中是允许存在多行值为NULL的数据存在的。 当然我们会认为这是My...
原文: The numberofdifferent valuesina table column.When queries refer to columns that have an associated index,the cardinalityofeach column influences which access method is most efficient.For example,fora columnwitha unique constraint,the numberofdifferent values is equal to the numberofrowsinthe ta...
In practice this makes a unique constraint on multiple columns completely useless once any of those columns is allowed to have null values. Nevertheless there are still a lot of valid use cases for these kind of situations. That's why I would still consider this a bug....
Cardinality can also apply to the number of distinct values present in multiple columns, as in a composite index. See Also column, composite index, index, index hint, persistent statistics, random dive, selectivity, unique constraint. 如何让sql强制使用索引...
Gap locking is not needed for statements that lock rows using a unique index to search for a unique row. (This does not include the case that the search condition includes only some columns of a multiple-column unique index; in that case, gap locking does occur.) --这一段话,要嘻嘻揣摩...
It's well and good for you to insist that multi-column constraints are supported, quite another to explain how to create them. The manual too is of no help, and neither is an internet search. So please spell it out: how does one create a uniqueness constraint on multiple columns?
constraint uk_name unique(name) ); mysql> insert into department1 values(1,'IT','技术'); Query OK,1 row affected (0.00sec) mysql> insert into department1 values(1,'IT','技术'); ERROR1062 (23000): Duplicate entry'IT'forkey'name' ...
InnoDB supports multiple granularity locking which permits coexistence of row-level locks and locks on entire tables. To make locking at multiple granularity levels practical, additional types of locks called intention locks are used. Intention locks are table-level locks in InnoDB that indicate which...
I have a number of such nullable columns and unique index combinations in my database schema and I'm trying to avoid having to create additional generated as columns for each case. Thanks, OC Subject Written By Posted Unique index/constraint multiple nulls ...