unique-key | 易学教程

How add unique key to existing table (with non uniques rows)

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问题 I want to add complex unique key to existing table. Key contains from 4 fields ( user_id , game_id , date , time ). But table have non unique rows. I understand that I can remove all duplicate dates and after that add complex key. Maybe exist another solution without searching all duplicate data. (like add unique ignore etc). UPD I searched, how can remove duplicate mysql rows - i think it's good solution. Remove duplicates using only a MySQL query? 回答1: You can do as yAnTar advised ALTER

MySQL - Meaning of “PRIMARY KEY”, “UNIQUE KEY” and “KEY” when used together while creating a table

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问题 Can anyone explain about the purpose of PRIMARY KEY , UNIQUE KEY and KEY , if it is put together in a single CREATE TABLE statement in MySQL? CREATE TABLE IF NOT EXISTS `tmp` ( `id` int(11) NOT NULL AUTO_INCREMENT, `uid` varchar(255) NOT NULL, `name` varchar(255) NOT NULL, `tag` int(1) NOT NULL DEFAULT '0', `description` varchar(255), PRIMARY KEY (`id`), UNIQUE KEY `uid` (`uid`), KEY `name` (`name`), KEY `tag` (`tag`) ) ENGINE=InnoDB AUTO_INCREMENT=1 ; How do I convert this query to MSSQL?

Difference between Key, Primary Key, Unique Key and Index in MySQL

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When should I use KEY , PRIMARY KEY , UNIQUE KEY and INDEX ? KEY and INDEX are synonyms in MySQL. They mean the same thing. In databases you would use indexes to improve the speed of data retrieval. An index is typically created on columns used in JOIN , WHERE , and ORDER BY clauses. Imagine you have a table called users and you want to search for all the users which have the last name 'Smith'. Without an index, the database would have to go through all the records of the table: this is slow, because the more records you have in your database, the more work it has to do to find the result. On

MySQL: ALTER IGNORE TABLE ADD UNIQUE, what will be truncated?

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问题 I have a table with 4 columns: ID, type, owner, description. ID is AUTO_INCREMENT PRIMARY KEY and now I want to: ALTER IGNORE TABLE `my_table` ADD UNIQUE (`type`, `owner`); Of course I have few records with type = \'Apple\' and owner = \'Apple CO\'. So my question is which record will be the special one to stay after that ALTER TABLE, the one with smallest ID or maybe the one with biggest as the latest inserted? 回答1: The first record will be kept, the rest deleted §§: IGNORE is a MySQL

difference between primary key and unique key

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I'm using mysql database. I have a confusion between primary key and unique key. Please help me where should I create primary and unique key. I mean in which situation we create unique key or primary key . Mr. KB Primary Key: There can only be one primary key in a table In some DBMS it cannot be NULL - e.g. MySQL adds NOT NULL Primary Key is a unique key identifier of the record Unique Key: Can be more than one unique key in one table Unique key can have NULL values It can be a candidate key Unique key can be NULL ; multiple rows can have NULL values and therefore may not be considered "unique

Unique keys in Entity Framework 4

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问题 An existing DB schema has unique, non-primary, keys, and some foreign keys that rely on them. Is it possible to define unique keys, which are not primary keys, in Entity Framework v4? How? 回答1: The Entity Framework 6.1 now supports uniques with both Data Annotations and Fluent API. Data Annotations (Reference) public class MyEntityClass { [Index(IsUnique = true)] [MaxLength(255)] // for code-first implementations public string MyUniqueProperty{ get; set; } } Fluent API (Reference) public

Unique Key constraints for multiple columns in Entity Framework

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I'm using Entity Framework 5.0 Code First; public class Entity { [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)] public string EntityId { get; set;} public int FirstColumn { get; set;} public int SecondColumn { get; set;} } I want to make the combination between FirstColumn and SecondColumn as unique. Example: Id FirstColumn SecondColumn 1 1 1 = OK 2 2 1 = OK 3 3 3 = OK 5 3 1 = THIS OK 4 3 3 = GRRRRR! HERE ERROR Is there anyway to do that? With Entity Framework 6.1, you can now do this: [Index("IX_FirstAndSecond", 1, IsUnique = true)] public int FirstColumn { get; set; } [Index("IX

Difference between Key, Primary Key, Unique Key and Index in MySQL

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问题 When should I use KEY , PRIMARY KEY , UNIQUE KEY and INDEX ? 回答1: KEY and INDEX are synonyms in MySQL. They mean the same thing. In databases you would use indexes to improve the speed of data retrieval. An index is typically created on columns used in JOIN , WHERE , and ORDER BY clauses. Imagine you have a table called users and you want to search for all the users which have the last name 'Smith'. Without an index, the database would have to go through all the records of the table: this is

INSERT … ON DUPLICATE KEY (do nothing)

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问题 I have a table with a unique key for two columns: CREATE TABLE `xpo`.`user_permanent_gift` ( `id` INT UNSIGNED NOT NULL AUTO_INCREMENT , `fb_user_id` INT UNSIGNED NOT NULL , `gift_id` INT UNSIGNED NOT NULL , `purchase_timestamp` TIMESTAMP NULL DEFAULT now() , PRIMARY KEY (`id`) , UNIQUE INDEX `user_gift_UNIQUE` (`fb_user_id` ASC, `gift_id` ASC) ); I want to insert a row into that table, but if the key exists, to do nothing! I don\'t want an error to be generated because the keys exist. I know

difference between primary key and unique key

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问题 I\'m using mysql database. I have a confusion between primary key and unique key. Please help me where should I create primary and unique key. I mean in which situation we create unique key or primary key . 回答1: Primary Key: There can only be one primary key in a table In some DBMS it cannot be NULL - e.g. MySQL adds NOT NULL Primary Key is a unique key identifier of the record Unique Key: Can be more than one unique key in one table Unique key can have NULL values It can be a candidate key