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
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...
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....
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...
Submitted: 2 Nov 2009 12:42Modified: 19 Nov 2009 19:12 Reporter: Karsten Wutzke Email Updates: Status: Not a Bug Impact on me: None Category: MySQL WorkbenchSeverity: S2 (Serious) Version: 5.2.6aOS: Any Assigned to: CPU Architecture: Any Tags: multiple columns, unique constraint View...
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...
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...
SHOW COLUMNS FROM tableName; DESC tableName; #第二种方式查询当前数据库所有表的详细信息SHOW TABLE STATUS FROM tableName; # 查询当前数据库所有表的详细信息 SHOW TABLE STATUS LIKE 'xxx%'; # 表名以xxx开头的表的信息 SHOW TABLE STATUS FROM tableName LIKE 'xxxx%'\G; # 加上 \G,查询结果按列...
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...
Many indexes with the same name were duplicated because multiple columns were combined, forming a PRIMARY KEY constraint. Duplicate-named indexes in PostgreSQL, unlike MySQL, are not permitted. Consequently, they are manually renamed to unique ones: 1 2 3 4 5 6 7 8 9 -- example of renaming...