I am currently doing some migration from MS Access to SQL Server. Access allows multiple Nulls in unique indexes where as SQL Server does not... I've been handling the migration by removing the indexes in SQL Server and adding filtered indexes:
CREATE UNIQUE NONCLUSTERED INDEX idx_col1_notnull
ON tblEmployee(col1)
WHERE col1 IS NOT NULL;
The problem I am having is that I am not sure how to implement a composite or multi-column "filtered" indexes... or if this is really possible as I've found no examples in researching it.
I do have an idea to implement it by creating filtered indexes like so:
CREATE UNIQUE NONCLUSTERED INDEX idx_col1col2_notnull
ON tblEmployee (col1, col2)
WHERE col1 IS NOT NULL
And then adding a second filtered index:
CREATE UNIQUE NONCLUSTERED INDEX idx_col2col1_notnull
ON tblEmployee (col1, col2)
WHERE col2 IS NOT NULL
But I'm not sure if this would even work let alone be the best method. Guidance in the right direction would be greatly appreciated.
You can add the following index to index only non nullable columns:
create table tblEmployee(col1 int, col2 int)
go
create unique nonclustered index idx_col1col2_notnull ON tblEmployee(col1,col2)
where col1 is not null and col2 is not null
go
--This Insert successeds
insert into tblEmployee values
(null, null),
(null, null),
(1, null),
(1, null),
(null, 2),
(null, 2)
--This Insert fails
insert into tblEmployee values
(3, 4),
(3, 4)
Also you can combine several indexes to keep unique resctrictions in all fields where a nullable field is null
CREATE TABLE MyTable (
[Idx_1] [int] NOT NULL,
[idx_2] [int] NOT NULL,
[idx_3] [int] NULL,
[no_index_field] [nvarchar](50) NULL
) ON [PRIMARY]
GO
CREATE UNIQUE NONCLUSTERED INDEX idx_3_fields
ON MyTable (Idx_1, Idx_2, Idx_3)
WHERE Idx_3 IS NOT NULL;
CREATE UNIQUE NONCLUSTERED INDEX idx_2_fields
ON MyTable (Idx_1, Idx_2)
WHERE Idx_3 IS NULL;
Related
I am building a database in POSTGRESQL, and I would like to create NOT NULL constraints for my columns, where one and only one column would be NOT NULL.
I have two columns in my table, site_id and buffer_result_id. Only one of these columns will have values.
alter table dt.analysis_result
add constraint ar_check check (site_id NOT NULL OR buffer_result_id NOT NULL);
The above code is just some pseudo-code to show my idea. How can I achieve this function?
You could use XOR expressed as:
alter table dt.analysis_result
add constraint ar_check check (
(site_id IS NOT NULL OR buffer_result_id IS NOT NULL)
AND NOT(site_id IS NOT NULL AND buffer_result_id IS NOT NULL)
);
db<>fiddle demo
More info: Exclusive OR - Equivalences
Demo:
CREATE TABLE analysis_result(site_id INT, buffer_result_id INT);
INSERT INTO analysis_result VALUES (NULL, NULL);
-- ERROR: new row for relation "analysis_result" violates check constraint "ar_check"
INSERT INTO analysis_result VALUES (1, 2);
-- ERROR: new row for relation "analysis_result" violates check constraint "ar_check"
INSERT INTO analysis_result VALUES (NULL, 2);
INSERT INTO analysis_result VALUES (1, NULL);
SELECT * FROM analysis_result
In Postgres, you can do this with a check constraint. I think the simplest method is to count the number of not null values:
alter table dt.analysis_result add constraint ar_check
check ( (site_id is not null)::int + (buffer_result_id is not null)::int = 1
);
This question already has answers here:
Add unique constraint to combination of two columns
(4 answers)
Closed 7 years ago.
I have one table with contains foreign key "columnA" column and one more column "columnB". I want to prevent adding same values in "columnB" but only for same value in "columnA"...
columnA columnB
1 'a'
1 'a' - this is not allowed
2 'a' - this is allowed
From my perspective only way to do that is by using trigger, but i suppose that there is a better, more elegant way to make this constraint. Do you know best way to make this logic?
A Unique constraint would work.
alter table TableName add constraint UQ_consrtaint unique(columnA, columnB);
That should do it.
It looks like you need to create a primary key like this:
DECLARE #DataSource TABLE
(
[A] TINYINT
,[B] CHAR
,PRIMARY KEY([A], [B])
);
INSERT INTO #DataSource ([A], [B])
VALUES (1, 'a'); -- ok
INSERT INTO #DataSource ([A], [B])
VALUES (2, 'a'); -- ok
INSERT INTO #DataSource ([A], [B])
VALUES (1, 'a'); -- error
It will give you the following error:
Msg 2627, Level 14, State 1, Line 14 Violation of PRIMARY KEY
constraint 'PK__#B1CFBEC__D86D1834E734E52B'. Cannot insert duplicate
key in object 'dbo.#DataSource'. The duplicate key value is (1, a).
in the case above.
Or unique constrain on the two columns:
DECLARE #DataSource TABLE
(
[A] TINYINT
,[B] CHAR
,UNIQUE ([A], [B])
);
ALTER TABLE tablename ADD UNIQUE uniqueconstraintname(columnA, columnB);
I have a data structure, where I have to store pairs of elements. Each pair has exactly 2 values in it, so we are employing a table, with the fields(leftvalue, rightvalue....).
These pairs should be unique, and they are considered the same, if the keys are changed.
Example: (Fruit, Apple) is the same as (Apple, Fruit).
If it is possible in an efficient way, I would put a database constraint on the fields, but not at any cost - performance is more important.
We are using MSSQL server 2008 currently, but an update is possible.
Is there an efficient way of achieving this?
Two solutions, both really about changing the problem into an easier one. I'd usually prefer the T1 solution if forcing a change on consumers is acceptable:
create table dbo.T1 (
Lft int not null,
Rgt int not null,
constraint CK_T1 CHECK (Lft < Rgt),
constraint UQ_T1 UNIQUE (Lft,Rgt)
)
go
create table dbo.T2 (
Lft int not null,
Rgt int not null
)
go
create view dbo.T2_DRI
with schemabinding
as
select
CASE WHEN Lft<Rgt THEN Lft ELSE Rgt END as Lft,
CASE WHEN Lft<Rgt THEN Rgt ELSE Lft END as Rgt
from dbo.T2
go
create unique clustered index IX_T2_DRI on dbo.T2_DRI(Lft,Rgt)
go
In both cases, neither T1 nor T2 can contain duplicate values in the Lft,Rgt pairs.
If you always store the values in order but store the direction in another column,
CREATE TABLE [Pairs]
(
[A] NVarChar(MAX) NOT NULL,
[B] NVarChar(MAX) NOT NULL,
[DirectionAB] Bit NOT NULL,
CONSTRAINT [PK_Pairs] PRIMARY KEY ([A],[B])
)
You can acheive exaclty what you want with one clustered index, and optimize your lookups too.
So when I insert the pair 'Apple', 'Fruit' I'd do,
INSERT [Pairs] VALUES ('Apple', 'Friut', 1);
Nice and easy. Then I insert 'Fruit', 'Apple',
INSERT [Pairs] VALUES ('Apple', 'Fruit', 0); -- 0 becuase order is reversed.
The insert fails because this is a primary key violation. To further illustrate, the pair 'Coconuts', 'Bananas' would be stored as
INSERT [Pairs] VALUES ('Bananas', 'Coconuts', 0);
For additional lookup performance, I'd add the index
CREATE NONCLUSTERED INDEX [IX_Pairs_Reverse] ON [Pairs] ([B], [A]);
If you can't control inserts to the table, it may be necessary to ensure that [A] and [B] are inserted correctly.
CONSTRAINT [CK_Pairs_ALessThanB] CHECK ([A] < [B])
But this may be an unnecessary performance hit, depending on how controlled your inserts are.
One way would be to create a computed column that combines the two values and put a unique constraint upon it:
create table #test (
a varchar(10) not null,
b varchar(10) not null,
both as case when a > b then a + ':' + b else b + ':' + a end persisted unique nonclustered
)
so
insert #test
select 'apple', 'fruit'
insert #test
select 'fruit', 'apple'
Gives
(1 row(s) affected)
Msg 2627, Level 14, State 1, Line 3
Violation of UNIQUE KEY constraint 'UQ__#test_____55252CB631EC6D26'. Cannot insert duplicate key in object 'dbo.#test'.
The statement has been terminated.
Unique constraint on two/more fields is possible but on their opposite no...
SQL Server 2005 Unique constraint on two columns
Unique constraint on multiple columns
How do I apply unique constraint on two columns SQL Server?
http://www.w3schools.com/sql/sql_unique.asp
Here is the sample table with 3 columns (ID, UNIQUE_VALUE, UNIQUE_GROUP_ID)
I want below records can be allowed:
(1, NULL, NULL)
(2, NULL, NULL)
or
(3, NULL, 7)
(4, 123, 7)
or (Note: this condition is not allowed in unique index nor unique constraint)
(5, NULL, 7)
(6, NULL, 7)
and these can't be allowed:
(7, 123, 7)
(8, 123, 7)
I created a unique index on last 2 columns, but only the first 2 examples can be allowed.
Is it possible to let db check the uniqueness of these 2 columns only when both are not null?
You want to only enforce uniqueness on the rows where both UNIQUE_VALUE and UNIQUE_GROUP_ID are not null. To do this, you can use a unique function-based index:
CREATE UNIQUE INDEX func_based_index ON the_table
(CASE WHEN unique_value IS NOT NULL
AND unique_group_id IS NOT NULL
THEN UNIQUE_VALUE || ',' || UNIQUE_GROUP_ID
END);
you can use the nvl function to avoid nulls and place a different value instead ,
create unique index func_idx on TEST_TABLE (nvl(UNIQUE_VALUE,1), UNIQUE_GROUP_ID);
the disadvantage is that your index will be larger and if you would like to search for null values you will have to use the nvl function in order to avoid table_access_full.
also all of the null values will be located under one branch in the index , so make sure your histograms are updated.
I Hope this will help you :)
I've had this come up a couple times in my career, and none of my local peers seems to be able to answer it. Say I have a table that has a "Description" field which is a candidate key, except that sometimes a user will stop halfway through the process. So for maybe 25% of the records this value is null, but for all that are not NULL, it must be unique.
Another example might be a table which must maintain multiple "versions" of a record, and a bit value indicates which one is the "active" one. So the "candidate key" is always populated, but there may be three versions that are identical (with 0 in the active bit) and only one that is active (1 in the active bit).
I have alternate methods to solve these problems (in the first case, enforce the rule code, either in the stored procedure or business layer, and in the second, populate an archive table with a trigger and UNION the tables when I need a history). I don't want alternatives (unless there are demonstrably better solutions), I'm just wondering if any flavor of SQL can express "conditional uniqueness" in this way. I'm using MS SQL, so if there's a way to do it in that, great. I'm mostly just academically interested in the problem.
If you are using SQL Server 2008 a Index filter would maybe your solution:
http://msdn.microsoft.com/en-us/library/ms188783.aspx
This is how I enforce a Unique Index with multiple NULL values
CREATE UNIQUE INDEX [IDX_Blah] ON [tblBlah] ([MyCol]) WHERE [MyCol] IS NOT NULL
In the case of descriptions which are not yet completed, I wouldn't have those in the same table as the finalized descriptions. The final table would then have a unique index or primary key on the description.
In the case of the active/inactive, again I might have separate tables as you did with an "archive" or "history" table, but another possible way to do it in MS SQL Server at least is through the use of an indexed view:
CREATE TABLE Test_Conditionally_Unique
(
my_id INT NOT NULL,
active BIT NOT NULL DEFAULT 0
)
GO
CREATE VIEW dbo.Test_Conditionally_Unique_View
WITH SCHEMABINDING
AS
SELECT
my_id
FROM
dbo.Test_Conditionally_Unique
WHERE
active = 1
GO
CREATE UNIQUE CLUSTERED INDEX IDX1 ON Test_Conditionally_Unique_View (my_id)
GO
INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (1, 0)
INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (1, 0)
INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (1, 0)
INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (1, 1)
INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (2, 0)
INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (2, 1)
INSERT INTO dbo.Test_Conditionally_Unique (my_id, active)
VALUES (2, 1) -- This insert will fail
You could use this same method for the NULL/Valued descriptions as well.
Thanks for the comments, the initial version of this answer was wrong.
Here's a trick using a computed column that effectively allows a nullable unique constraint in SQL Server:
create table NullAndUnique
(
id int identity,
name varchar(50),
uniqueName as case
when name is null then cast(id as varchar(51))
else name + '_' end,
unique(uniqueName)
)
insert into NullAndUnique default values
insert into NullAndUnique default values -- Works
insert into NullAndUnique default values -- not accidentally :)
insert into NullAndUnique (name) values ('Joel')
insert into NullAndUnique (name) values ('Joel') -- Boom!
It basically uses the id when the name is null. The + '_' is to avoid cases where name might be numeric, like 1, which could collide with the id.
I'm not entirely aware of your intended use or your tables, but you could try using a one to one relationship. Split out this "sometimes" unique column into a new table, create the UNIQUE index on that column in the new table and FK back to the original table using the original tables PK. Only have a row in this new table when the "unique" data is supposed to exist.
OLD tables:
TableA
ID pk
Col1 sometimes unique
Col...
NEW tables:
TableA
ID
Col...
TableB
ID PK, FK to TableA.ID
Col1 unique index
Oracle does. A fully null key is not indexed by a Btree in index in Oracle, and Oracle uses Btree indexes to enforce unique constraints.
Assuming one wished to version ID_COLUMN based on the ACTIVE_FLAG being set to 1:
CREATE UNIQUE INDEX idx_versioning_id ON mytable
(CASE active_flag WHEN 0 THEN NULL ELSE active_flag END,
CASE active_flag WHEN 0 THEN NULL ELSE id_column END);