In my table I want to have an ID column that is going to be unique and it is going to be autoincremented. I want to start from 0.
How can I implement it?
Basically, create a column of type INTEGER PRIMARY KEY or a column called ROWID, then don't specify the value when inserting a row.
Full details are on the SQLite page on Autoincrement.
Related
Consider the following table
I want to add constraint which says "The ID numbers must be unique". This means that I cannot have the same Value when the Type is ID_NUMBER, but I can have the same value with another Type (e.g USERNAME).
How can achieve this ? I was thinking about some kind of check constraint on the two columns but I don't know how to provide "ID_NUMBER" for the value of one of the columns in the constraint.
You can create a filtered unique index:
create unique index unq_table_value on table(value) where field = 'ID_NUMBER';
I have a table with an auto-increment primary key column; I want to fetch the items of this table order by the time when the record is inserted. I wonder is it necessary to have a dedicated insert_time column for that purpose?
If you want to know which records came before others you can just use the primary key. If you want to know the actual time of the record, you will need another column.
You don't need to add insert_time column. You can order your data with auto-increment column. Last inserted row have max number.
Is there any way to simply add not null unique column to existing table. Something like default = 1++ ? Or simply add unique column?
I tried to add column and then put unique contrain but MS SQL says that:
The CREATE UNIQUE INDEX statement terminated because a duplicate key was found for the object name (...) The duplicate key value is ( < NULL > ).
Any way to simply add column to existing working table with unique constrain? Should MS SQL really think that null IS a value?
Add not null column with some default.
Update column to be a sequential integers (see row_number() function)
Add UNIQUE constraint or UNIQUE index over new column
You can add IDENTITY column to a table (but from question it is not clear if you need it or not).
IDENTITY is all you need:
ALTER TABLE TestTable
ADD ID INT IDENTITY(1, 1)
If you want an autonumber, you can add an identity.
If you need to populate the values yourself, you add the column allowing nulls, update the values, check to make sure they are unique and that there are no nulls, then add the unique constraint and trhe not null property. It is best to do this during a maintenance window when no one else would be changing data in that table.
I have column having name id in my table. id is the primary key in table. I want to know that is it necessary to have separate column for id as it is primary key in my table.
Not necessary to have a separate column, you could have an existing column as primary key if it can identify each record uniquely..
Any field or combination of fields can be a primary key if:
The values in those fields are always non-null.
The records with values in those fields are unique.
Those fields are immutable. That is, you won't change the values of those fields
after the record is created.
It's always better to keep things simple. If you already have a column that identifies the record it's just fine - don't add a new one.
There is also something called composite primary keys. You can use it if a combination of 2 or more columns always creates a unique sequence. Than you don't really need the 'Id' column. The truth though is some frameworks don't like this approach.
In your case the column you already have should be sufficient.
The PRIMARY KEY constraint uniquely identifies each record in a database table and if your table already contain that column then u don't need to add another column.
I'm creating a table that needs to have 2 columns. The first column can't be repeated. The thing is, I will insert the value of the first column. How do I create this column?
SQLServer 2005
Make the first column the primary key of the table.
Set the column as a primary key. I doesn't have to be an identity column to has a PK.
Create it the same way you would any other column: create table sometable (column1 varchar(10), column2 varchar(20)) or whatever.
Do you mean: How can you get the database to force it to be unique? Either declare it to be the primary key, or create a unique index on the column.
Perhaps you're thinking that a primary key must be auto-generated? There's no such rule. Whether you invent the value yourself or use an autonumber feature has nothing to do with whether a field can be a primary key.
Why not just put a unique constraint on it:
ALTER TABLE <table_name>
ADD CONSTRAINT <constraint_name>
UNIQUE(<column_name>)