unique-key

问题 Very basic question; I'm very new to SQL and trying to decipher an example data base. In the below create table code, why does the define primary key syntax reference only the 'id' column once in parentheses but the unique key definition references the 'category' column twice? both before and within the parentheses. Seems like there is a simple answer to this but cant track one down: CREATE TABLE `categories` ( `id` SMALLINT NOT NULL AUTO_INCREMENT, `category` VARCHAR(30) NOT NULL, PRIMARY

问题 I know what the term "SuperKey" in SQL stands for, but I cannot understand a specific thing and I would like some help. In a table with no data , is there a superkey? In any table, will there always exists one? 回答1: TL;DR "Superkey" is a RM (Relational Model of Data) term. There's no standard use in SQL. The superkeys of an SQL table might reasonably informally be said to be the column sets that you could declare primary key or unique not null , plus maybe {} when a table holds at most one

问题 For a project with offline storage, I created a database with a primary key for the id and also a unique field holding the date. When I delete an entry, I can not create a new one with the same date the deleted entry had. What can I do to get the date out of the index when I delete that entry? This is how I create Table: CREATE TABLE IF NOT EXISTS userdata (id INTEGER PRIMARY KEY ASC, entrydate DATE UNIQUE, strength, comments I wonder if I need to add something to tell the DB server to allow

问题 I have a table in SQL Server 2005 with a foreign key, and I want that foreign key to be a unique value or null. I've set it up as a unique key, but it won't allow me to have multiple nulls in the same table. Is it possible to do what I want? 回答1: This is a long time complaint about SQL Server's Unique constraints/indexes. The best solution is to create a view with schemabinding and then put a unique index on that column: Create View dbo.MyUniqueColView With SchemaBinding As Select

问题 I am new to SQL Server and while learning about clustered index, I got confused! Is unique key clustered or a non-clustered index? Unique key holds only unique values in the column including null, so according to this concept, unique key should be a clustered index, right? But when I went through this article I got confused MSDN When you create a UNIQUE constraint, a unique nonclustered index is created to enforce a UNIQUE constraint by default. You can specify a unique clustered index if a

问题 I want to retrieve the unique ID of a machine. Like others I also did a lot of research, and found none of the process of Unique ID generation works perfectly. For Motherboard Serial Number (ID): It is Unique; it can't be changed. However, it may not be found in some machines if Manufacturers didn't put information on Memory Location. Then I found it gives no Unique Id .. lol Similarly for "Processor ID", "BIOS ID". Afraid to use other hardware information of PC. MAC and Windows Product ID

问题 I want to retrieve the unique ID of a machine. Like others I also did a lot of research, and found none of the process of Unique ID generation works perfectly. For Motherboard Serial Number (ID): It is Unique; it can't be changed. However, it may not be found in some machines if Manufacturers didn't put information on Memory Location. Then I found it gives no Unique Id .. lol Similarly for "Processor ID", "BIOS ID". Afraid to use other hardware information of PC. MAC and Windows Product ID

问题 I tried to find it out in google but not satisfactory answer is given out there. Can anybody explain the solid difference. actually if Primary key is used to select data uniquely then what is the need of Unique key? When should I use a Primary key and when to use a Unique key? 回答1: Primary Key and Unique Key are used for different things - understanding what they are for will help you decide when to use them. The primary key is used to identify a row of data in a table. It is used whenever

问题 i want to make a table in MySQL server with mediumtext column as UNIQUE KEY CREATE TABLE `parts` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` mediumtext NOT NULL, `display_status` int(11) NOT NULL, UNIQUE KEY `name` (`name`), PRIMARY KEY (`id`) ) ENGINE=MyISAM DEFAULT CHARSET=utf8; but this made an error BLOB/TEXT column 'name' used in key specification without a key length when I change the type of `name` to varchar .. it works! can you tell if i can to make text column as UNIQUE KEY 回答1:

问题 Could someone clarify what is the purpose of having unique index without unique constraint (Oracle)? For example, create table test22(id int, id1 int, tmp varchar(20)); create unique index idx_test22 on test22(id); insert into test22(id, id1, tmp) values (1, 2, 'aaa'); // ok insert into test22(id, id1, tmp) values (1, 2, 'aaa'); // fails, ORA-00001: unique // constraint (TEST.IDX_TEST22) violated So far it looks like there is a constraint. But create table test33(id int not null primary key,