primary-key

If I define a column as a primary key in MySql, is it also unique key by default or do I need to also define it as unique key (in case I want it to be unique)? I saw this question What is the difference b/w Primary Key and Unique Key that explain the difference between the two but doesn't exactly answer my question. Does PK is UK by default or I need to explicitly define it. Primary key is always unique in every SQL. You dont have to explicitly define it as UNIQUE. On a side note: You can only have onePrimary key in a table and it never allows null values. Also you can have only one primary

In MSSQL I have a table created like this: CREATE TABLE [mytable] (fkid int NOT NULL, data varchar(255) CONSTRAINT DF_mytable_data DEFAULT '' NOT NULL); ALTER TABLE [mytable] ADD CONSTRAINT PK_mytable_data PRIMARY KEY (fkid, data); Now I want to increase the length of the 'data' column from 255 to 4000. If I just try: ALTER TABLE [mytable] ALTER COLUMN data varchar(4000); Then I get this error: The object 'PK_mytable_data' is dependent on the column 'data' If I try this: ALTER TABLE [mytable] DROP CONSTRAINT PK_mytable_data; ALTER TABLE [mytable] ALTER COLUMN data varchar(4000); ALTER TABLE

Is it bad to have text as a primary key in an SQLite database? I heard that it's bad for performance reasons, is this true? And will the rowid be used as the actual primary key in such a case? Is it bad to have text as a primary key in an SQLite database? I heard that it's bad for performance reasons, is this true? I never heard that somebody used string as primary key in table. For me (and I honestly hope also for others) very "ugly" practise with very low performance. If you'll use string as primary key you needs to think about a "few" things: Will be combination of 3 symbols enough? Or

I have lots of situations where I'd like to have a non-autoincrement primary key when using Rails. Example: I have one-to-one relationship between A and B. B describes some specific features added to A thus can't exist without A. So we have: A has one B B belongs to A The natural thinking would be having B.A_id as primary key. So I tried create_table b, :id=>false in migration and set_primary_key :a_id in B's model, but it doesn't create actual primary keys in the database. And I want them (as well as foreign keys), as the database will be used not only by this Rails app. If I create primary

I realize this question is very likely to have been asked before, but I've searched around a little among questions on StackOverflow, and I didn't really find an answer to mine, so here goes. If you find a duplicate, please link to it. For some reason I prefer to use Guid s ( uniqueidentifier in MsSql) for my primary key fields, but I really don't know why this would be better. In many of tutorials I've walked myself through lately an automatically incremented int has been used. I can see pro's and cons with both: A Guid is always of the same size and length, and there is no reason to worry

I am importing Tables from a Oracle DataBase, using a VB.NET dataAdapter. I use the "fill" command to add the imported data to a DataSet. How is it possible to define a specific column of a DataTable as PrimaryKey, after the DataTable is already filled with data? You can set the primary key of a table by: Dim table As New DataTable() table.Columns.Add(New DataColumn("MyColumn")) Dim primaryKey(1) As DataColumn primaryKey(1) = table.Columns("MyColumn") table.PrimaryKey = primaryKey To be able to use the primary key, you need to ensure that all values for the given column are unique. I primarily