I would like to know if there is a way in sql to allow a column to have any value it wants for its attributes, but only one of them can be X value.
More specifically for easier explanations let's say i have:
create table status( current_status varchar(100), id integer )
what constraint should i add so i can only have up to one value of current_status='ACCEPTED' ?
EDIT:REMOVED mysql TAG
Partial index in Postgresql
create unique index index_accepted on status (current_status)
where current_status = 'ACCEPTED'
Related
I searched but only could found partial answer to this question
The goal would be here to create a new ID column on an existing table.
This new column would be the primary key for the table and I simply want it to be filled with integer values from 1 to number of rows.
What would be the query for that?
I know I have to first alter table to create the new column :
ALTER TABLE <MYTABLE> ADD (ID INTEGER);
Then I could use the series generator :
INSERT INTO <MYTABLE.ID> SELECT SERIES_GENERATE_INTEGER(1,1,(number of rows));
Once the column is filled I could use this line:
ALTER TABLE <MYTABLE> ADD PRIMARY KEY ("ID");
I am sure there is an easier way to do this
You wrote that you want to add a "new ID column to an existing table" and fill it with unique values.
That's not a "standard" operation in any DBMS, as the usual assumption is that records are created with a primary key and not retro fitted.
Thus, "ease" of operation for this is relative to what else you want to do.
For example, if you want to continue using this ID as a primary key for further operations, then using a once-off generator function like the SERIES_GENERATE_INTEGER or a query won't be very helpful since you have to avoid duplicates of already existing values.
Two, relatively easy, options come to mind:
Using a sequence:
create sequence myid;
update <table> set ID = myid.nextval;
And for succeeding inserts:
insert into <table> (id, ..., ...) VALUES (myid.nextval, ..., ...) ;
Note that this generates a value for every existing record and not a predefined set of size X.
Using a GUID
By using a GUID you generate a unique value every time you call the 'SYSUUID' function in SAP HANA. check docu here
Something like
update <table> set ID = SYSUUID;
should do the trick here.
Subsequent inserts would simply call the function for values of ID.
How can i create a table, that allows only to put data in NAME, if the data matches with the data that i want to be allowed in NAME. So like Bla1 or Bla2.
CREATE TABLE Table1 (
NAME VARCHAR(23)
NAME has to be one of them: ('Bla1', 'Bla2')
)
The best way to do it is probably to have a second table with all the allowed names in it, and making a FOREIGN KEY from the name field in your Table1 to the name field in that other table. That'll automatically fail any insert queries for which the name is not contained in the list of allowed names.
This has an advantage over things like ENUM and such in that it does not require you to rebuild your table (which is a very expensive operation) every time you want to allow another name and it also allows you to later add additional related info to each name by adding it to the other table.
Here's a great article on why using a foreign key is much better than using enums or other such checks in the table itself: http://komlenic.com/244/8-reasons-why-mysqls-enum-data-type-is-evil/
Try this:
CREATE TABLE Table1 (
name VARCHAR(23) CHECK( name IN ('Bla1','Bla2') )
);
I have a table with a column which contains a 'valid until' Date and I want to make sure that this can only be set to null in a single row within the table. Is there an easy way to do this?
My table looks like this (postgres):
CREATE TABLE 123.myTable(
some_id integer NOT NULL,
valid_from timestamp without time zone NOT NULL DEFAULT now(),
valid_until timestamp without time zone,
someString character varying)
some_id and valid_from is my PK. I want nobody to enter a line with a null value in column valid_until if there is already a line with null for this PK.
Thank you
In PostgreSQL, you have two basic approaches.
Use 'infinity' instead of null. Then your unique constraint works as expected. Or if you cannot do that:
CREATE UNIQUE INDEX null_valid_from ON mytable(someid) where valid_until IS NULL
I have used both approaches. I find usually the first approach is cleaner and it allows you to use range types and exclude constraints in newer versions of PostgreSQL better (to ensure no two time ranges overlap based on a given given someid), bt the second approach often is useful where the first cannot be done.
Depending on the database, you can't have null in a primary key (I don't know about all databases, but in sql server you can't). The easiest way around this I can think of is to set the date time to the minimum value, and then add a unique constraint on it, or set it to be the primary key.
I suppose another way would be to set up a trigger to check the other values in the table to see if another entry is null, and if there is one, don't allow the insert.
As Kevin said in his answer, you can set up a database trigger to stop someone from inserting more than one row where the valid until date is NULL.
The SQL statement that checks for this condition is:
SELECT COUNT(*)
FROM TABLE
WHERE valid until IS NULL;
If the count is not equal to 1, then your table has a problem.
The process that adds a row to this table has to perform the following:
Find the row where the valid until value is NULL
Update the valid until value to the current date, or some other meaningful date
Insert the new row with the valid until value set to NULL
I'm assuming you are Storing Effective-dated-records and are also using a valid from date.
If so, You could use CRUD stored procedures to enforce this compliance. E.G the insert closes off any null valid dates before inserting a new record with a null valid date.
You probably need other stored procedure validation to avoid overlapping records and to allow deleting and editing records. It may be more efficient (in terms of where clauses / faster queries) to use a date far in the future rather than using null.
I know only Oracle in sufficient detail, but the same might work in other databases:
create another column which always contains a fixed value (say '0') include this column in your unique key.
Don't use NULL but a specific very high or low value. I many cases this is actually easier to use then a NULL value
Make a function based unique key on a function converting the date including the null value to some other value (e.g. a string representation for dates and 'x' for null)
make a materialized view which gets updated on every change on your main table and put a constraint on that view.
select count(*) cnt from table where valid_until is NULL
might work as the select statement. And a check constraint limiting the cnt value to the values 0 and 1
I would suggest inserting to that table through an SP and putting your constraint in there, as triggers are quite hidden and will likely be forgotten about. If that's not an option, the following trigger will work:
CREATE TABLE dbo.TESTTRIGGER
(
YourDate Date NULL
)
CREATE TRIGGER DupNullDates
ON dbo.TESTTRIGGER
FOR INSERT, UPDATE
AS
DECLARE #nullCount int
SELECT #nullCount = (SELECT COUNT(*) FROM TESTTRIGGER WHERE YourDate IS NULL)
IF(#NullCount > 1)
BEGIN
RAISERROR('Cannot have Multiple Nulls', 16, 1)
ROLLBACK TRAN
END
GO
Well if you use MS SQL you can just add a unique Index on that column. That will allow only one NULL. I guess that if you use other RDBMS, this will still function.
There is a need to build constraint on the column that guarantees that only one value in all rows is 1 and all the others are 0.
Solution with triggers exists but I would like to have something built in.
Is such thing possible at all?
Edit
Actually I just noticed you are on SQL Server 2008 you could use a filtered index for this
CREATE UNIQUE NONCLUSTERED INDEX UIX ON YourTable (col) where col = 1
Original Answer
The easiest way would probably be to store this one special pk in a separate one row table. The no more than one row aspect can be enforced with check constraints.
CREATE TABLE OneRowTable
(
lock CHAR(1) DEFAULT 'X' NOT NULL PRIMARY KEY CHECK (lock = 'X'),
OtherTablePK int
);
Otherwise assuming you might have an id field comprised of positive integers you could add a computed column with the following definition
case when col=1 then -1 else id end
and add a unique constraint to that.
Is it possible to create a table which has a check constraint on one of the column whose value lies within the result set given by another sql query
eg.
create table tablename
(
name varchar(10),
rollno int
)check rollno in (select rollno from anotherDatabase.TableName,candidateRoll)
or any thing like that.
I dont have to use it anywhere but still want to know.
If you can't achieve what you want with a foreign key reference, so you can if you wrap the SELECT statement in a function call.
Your check constraint expression may look something like:
(dbo.SomeFunction([col1]) != 0)
The function might look like this (assuming the column is a varchar):
create function dbo.SomeFunction(#arg varchar(max))
returns bit
as
begin
return
(
select count(*) from SomeOthertable where col2 = #arg
)
end
EDIT (2010/06/9): Regarding Anthony's comment, my testing has shown that a count(*) value of greater than 1 is still returned as 1. So it would seem that the function is okay, even though it should probably explicitly return 1 or 0. Or, if you are interested in the actual rowcount, change the return type from BIT to INT.
Yes: foreign key for same database links
create table tablename
(
name varchar(10),
rollno int FOREIGN KEY (candidateRoll) REFERENCES OtherTableName (candidateRoll)
)
If it's a different database then use code e.g. insert via stored proc or enforce via a trigger