[MySQL Documentation]( [MySQL - Creating unique keys on multiple columns
A multiple-column index can be considered a sorted array, the rows of which contain values that are created by concatenating the values of the indexed columns. 联合索引是多列按照次序一列一列比较大小,拿idx_book_id_hero_name这个联合索引来说,先比较book_id,book_id小的排在左边,book_id大的排在...
For example, if you have a unique key with two columns, the first column can be interpreted as a "namespace". The second column must be unique in any namespaces. Multiple NULL values in a namespace is allowed, remember back to a single column rule. This is valid: namespace | name -...
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 table.If...
UNIQUE KEY (UK) 标识该字段的值是唯一 AUTO_ 标识该字段的值自动增长(整数类型,而且为主键) DEFAULT 为该字段设置默认值 UNSIGNED 无符号 ZEROFILL 使用0填充 说明: 1. 是否允许为空,默认NULL,可设置NOT NULL,字段不允许为空,必须赋值 2. 字段是否有默认值,缺省的默认值是NULL,如果插入记录时不给字段...
Match the full value(全值匹配):A match on the full key value specifies values for all columns in the index. For example, this index can help you find a person named Cuba Allen who was born on 1960-01-01. Match a leftmost prefix(匹配最左前缀):This index can help you find all people...
8.3.5 Multiple-Column Indexes MySQL can create composite indexes (that is, indexes on multiple columns). An index may consist of up to 16 columns. For certain data types, you can index a prefix of the column (seeSection 8.3.4, “Column Indexes”). ...
I made a unique key out of two columns to use as my primary key. I want one of the columns to be a foreign key as well, but I get an error when I try to do that. I have a table (drawing) that stores drawings with the primary key dwgID. I want to make a table (drawingrev...
复制表结构和记录 (key不会复制: 主键、外键和索引) 命令格式:create table 新表名 select * from 源表名; 实例:下面是将service的表结构和数据复制到new_service表中。 mysql> create table new_service select * from service; 提示:复制表之后,请用show tables、desc和select命令查看表的相关信息。
data_id is the unique key, but this code gives me the column i want with 1 or NULL as the result which is correct in that if any of the columns has a value over 50 it has the result 1, but some columns should show 2, 3,4 or 5 as i want the count. ...