whats is the difference between these two statements to make a Column Unique?
ALTER TABLE [TABLE]
ADD CONSTRAINT [Cons_name] UNIQUE (Columname);
ALTER TABLE [TABLE]
ADD UNIQUE (Columname);
Also, there is any difference when I want to add a composite unique as (column1, column2,...)?
Related
I'm using psql and I want to change one of the columns of my table.
At the moment this column is a foreign key of Table 2 but I would like to make it point to Table 3.
Is this possible or should I delete the column and add a new one?
There's no need to add & remove the column. You can remove/disable the constraint to one table and add it for the other table.
The command for doing the former is:
alter table Table1 drop constraint if exists name_of_constraint_on_Table_1_column
The command for doing the latter is:
alter table Table1
add constraint name_of_constraint_on_Table_1_column
foreign key (column) references Table3 (other_column) match full
You need to find the name of the foreign key constraint if you haven't named it explicitly. You can do so via the \d command:
\d Table1
You should read about alter table cause there's a lot of things you can do to change the table.
I've inserted several rows into a table, using duplicates in a column (theId) with a unique key constraint. I did this by setting IDENTITY_INSERT to off then on.
I tried this same technique to update, since I need to change those dupe values but it isn't working:
SET IDENTITY_INSERT mytable OFF
update mytable set
theId = 5
WHERE mytableId in (40, 41)
SET IDENTITY_INSERT mytable ON
Error:
Violation of UNIQUE KEY constraint 'XI_mytale_mytableId_othercolumn_U'. Cannot insert duplicate key in object 'dbo.mytable'.
Any ideas how this can be done with an UPDATE?
I think you are confusing with An Identity Column where you managed to add values manually after Setting SET IDENTITY_INSERT mytable ON; with a Column with Unique Constraint Defined on it. A column with Unique Constraint will never allow you to add duplicate values.
If you do want to be able to add Duplicate values in a column with a unique constraint you can simple drop the unique constraint as why have a unique constraint when you dont have unique values.
You can droo the Unique Constraint using following statement,
ALTER TABLE TableName
DROP CONSTRAINT uc_ConstraintName
I am adding a new column in an existing table with preloaded data. This column uses a primary key from another table and I want to default this to 5. I have tried the following code:
ALTER TABLE group
ADD group_type INT
GO
ALTER TABLE group
ADD CONSTRAINT FK_group_type DEFAULT 5 FOR group_type
GO
I was expecting on alter of the group table then all the values will be filled with 5 but instead its NULL. What am I doing wrong?
First of all, adding a DEFAULT constraint (in it's own SQL statement) to a column does not effect existing data in that column. It only effects new INSERTS to that table which do not provide a value for that column.
Second, you haven't created a FOREIGN KEY constraint here.
EDIT:
Here would be one way to create the FK correctly
ALTER TABLE group
ADD group_type_id INT
GO
ALTER TABLE group
ADD CONSTRAINT fk_groupType FOREIGN KEY (group_type_id)
REFERENCES group_type (group_type_id)
This worked for me, it set the foreign key constraint, default value, and updated existing table records all in a single ALTER TABLE statement. I'm using a SQL Azure database (via SQL Management Studio), so, YMMV.
ALTER TABLE Group
ADD GroupTypeId int NOT NULL
CONSTRAINT DK_GroupTypeId DEFAULT (5) WITH VALUES
CONSTRAINT FK_GroupTypeId FOREIGN KEY
REFERENCES [dbo].[GroupType] (GroupTypeId)
GO
It took a while to run, but a subsequent select, showed the rows had the correct default value for those columns.
Disclaimer: I edited the above query from my table / key names to yours without re-testing it, so you may want to double check it for any typos or other mismatches; The syntax should be the same though.
You can use:
alter table [group]
add group_type int constraint df_group_type default (5) with values
However, it doesn't seem a good idea to use constant as a default value for a column, which is supposed to be FK column.
It seems, that may be what actually you are trying to do is following:
alter table [group] add column group_type int
GO
update [group] set group_type = (select id from group_type where desc ='typeA')
GO
alter table [group] add constraint FK_group_grouptype foreign key (group_type) references group_type (id)
GO
Adding default constraint will affect existing rows if you add new not nullable column to table.
ALTER TABLE group
ADD group_type INT NOT NULL
CONSTRAINT DK_group_type DEFAULT 5
GO
Newbie in need of help.
I'm creating a small database.
I want to constrain data in a couple of tables - Lender and Pingtree. The ProviderType table contains lookup data and contains either 'Lender' or 'Pingtree'. How can I modify this structure so that the Lender table only can contain Lender types and Pingtree, Pingtree types?
Guessing that ProviderTypeID is the column in the Provider table that distinguishes between the two types, then you must add this same column to both the Lender and Pingtree tables, add a suitable key (if it doesn't already exist) in Provider on ID, ProviderTypeId, and then add a composite FOREIGN KEY constraint to the Lender and Pingtree tables that include these columns.
While this may sound like a drag, it is a known pattern called supertyping/subtyping. When the supertype (Provider) can be multiple subtypes, you don't need the TypeId column. But when the subtypes are mutually exclusive, this is what you must do.
It might look something like this:
ALTER TABLE dbo.Lender ADD ProviderTypeId tinyint NOT NULL
CONSTRAINT DF_Lender_ProviderTypeID DEFAULT (1)
CONSTRAINT CK_Lender_ProviderTypeID_Is_Lender CHECK (ProviderTypeID = 1);
ALTER TABLE dbo.Pingtree ADD ProviderTypeId tinyint NOT NULL
CONSTRAINT DF_Pingtree_ProviderTypeID DEFAULT (2)
CONSTRAINT CK_Pingtree_ProviderTypeID_Is_Pingtree CHECK (ProviderTypeID = 2);
-- Any of a PK, UNIQUE constraint, or unique index will do
ALTER TABLE dbo.Provider ADD CONSTRAINT UQ_Provider_ID_ProviderTypeID
UNIQUE (ID, ProviderTypeID);
ALTER TABLE dbo.Lender DROP CONSTRAINT FK_Lender_ProviderId;
ALTER TABLE dbo.Lender ADD CONSTRAINT FK_Lender_ProviderId_ProviderTypeId
FOREIGN KEY (ID, ProviderTypeID) REFERENCES dbo.Provider (ID, ProviderTypeID);
ALTER TABLE dbo.PingTree DROP CONSTRAINT FK_PingTree_ProviderId;
ALTER TABLE dbo.PingTree ADD CONSTRAINT FK_PingTree_ProviderId_ProviderTypeId
FOREIGN KEY (ID, ProviderTypeID) REFERENCES dbo.Provider (ID, ProviderTypeID);
If written correctly (specifying the column list on INSERT) your stored procedures and application SQL code should not have to change.
I am student of RDBMS.
I have very basic question let say I have one existing Table in SQL server. What will be script to alter table.
Drop Column 'RowId' if exist.
Drop contraint if exist.
Add one new column 'RowId' into table.
Make this column as primary key.
Autoincrement type int.
In SQL Server 2005 or newer, you could use this script:
-- drop PK constraint if it exists
IF EXISTS (SELECT * FROM sys.key_constraints WHERE type = 'PK' AND parent_object_id = OBJECT_ID('dbo.YourTable') AND Name = 'PK_YourTable')
ALTER TABLE dbo.YourTable
DROP CONSTRAINT PK_YourTable
GO
-- drop column if it already exists
IF EXISTS (SELECT * FROM sys.columns WHERE Name = 'RowId' AND object_id = OBJECT_ID('dbo.YourTable'))
ALTER TABLE dbo.YourTable DROP COLUMN RowId
GO
-- add new "RowId" column, make it IDENTITY (= auto-incrementing)
ALTER TABLE dbo.YourTable
ADD RowId INT IDENTITY(1,1)
GO
-- add new primary key constraint on new column
ALTER TABLE dbo.YourTable
ADD CONSTRAINT PK_YourTable
PRIMARY KEY CLUSTERED (RowId)
GO
Of course, this script may still fail, if other tables are referencing this dbo.YourTable using foreign key constraints onto the pre-existing RowId column...
Update: and of course, anywhere I use dbo.YourTable or PK_YourTable, you have to replace those placeholder with the actual table / constraint names from your own database (you didn't mention what they were, in your question.....)
Note: this answer was added before questions update
Add new column (note: you can only have one IDENTITY column per table)
Drop old primary key
Add new primary key
Drop old column if needed
Sample script:
CREATE TABLE whatever (
OldPKColumn uniqueidentifier NOT NULL,
CONSTRAINT PK_whatever PRIMARY KEY (OldPKColumn)
)
ALTER TABLE whatever
ADD RowId int NOT NULL IDENTITY (1,1);
ALTER TABLE whatever
DROP CONSTRAINT PK_whatever;
ALTER TABLE whatever WITH CHECK
ADD CONSTRAINT PK_whatever PRIMARY KEY CLUSTERED (RowId);
ALTER TABLE whatever
DROP COLUMN oldPKcolumn;
And a random thought... are you trying to reset an IDENTITY column?
If so, then use DBCC CHECKIDENT
Just a comment to improve these great answers (can't use comments yet - I'm one reputation point away from that privilege) and as future reference for myself:
A new IDENTITY (autonumber) column can be added and made the primary key in a single statement as well:
ALTER TABLE [TableName] ADD [ColumnName] int IDENTITY PRIMARY KEY;
I prefer not to bother with constraint names when it doesn't help.
You can specify seed (and increment) values between parantheses after the IDENTITY keyword.