Can I add a CHECK in my SQL CREATE TABLE script? - sql

Question

I created this table for my database.
CREATE TABLE Reservation
(
Reservation_Name SERIAL UNIQUE PRIMARY KEY,
User VARCHAR(64) DEFAULT 'Member', FOREIGN KEY(User) REFERENCES User(Email)
ON UPDATE CASCADE
ON DELETE SET DEFAULT,
Location INT, FOREIGN KEY(Location) REFERENCES Place(Id_Location)
ON UPDATE CASCADE
ON DELETE NO ACTION,
Start_Date DATE NOT NULL,
Check_In TIME(1) DEFAULT '10:00:00',
End_Date DATE NOT NULL,
Check_Out TIME(1) DEFAULT '18:00:00',
CHECK(Start_Date >= End_Date),
Deleted BOOLEAN DEFAULT FALSE
);
How can I insert a Check that doesn't allow to add a reservation if there's already another one with the same Start_Date and the same End_Date end the same location?

Answer 1

You can use an exclusion constraint:
CREATE EXTENSION btree_gist;
ALTER TABLE reservation ADD EXCLUDE USING gist (
location WITH =,
daterange(start_date, end_date, '[]') WITH &&
);
The extension is required so that you can create a GiST index on an integer column, and && is the "overlaps" operator for range types.

Answer 2

You can symply add a constraint to the table.
Alter Table Reservations Add Constraint unique_reservation Unique(Location,StartDate,EndDate);
You will need a trigger for this. Look at the above code :
CREATE TRIGGER no_overlap
BEFORE INSERT
ON Reservation FOR EACH ROW
BEGIN
SET #overlaps = ( SELECT count(*) FROM Reservation WHERE ( ( NEW.Start_Date >= Start_Date AND NEW.Start_Date <= End_Date AND NEW.Location = Location) || ( NEW.End_Date >= Start_Date AND NEW.End_Date <= End_Date AND NEW.Location = Location)));
IF #overlaps > 0 THEN
SIGNAL SQLSTATE '45000' SET
MYSQL_ERRNO = 31000,
MESSAGE_TEXT = 'Unable to insert an overlapping reservation';
END IF;
END;
INSERT INTO Reservation (Location,Start_Date,End_Date) VALUES(1,'2020-12-13','2020-12-16');
INSERT INTO Reservation (Location,Start_Date,End_Date) VALUES(1,'2020-12-14','2020-12-17');
The first insert will succeed while the second one will fail with the corresponding error message if the dates overlap :
SQL Error [31000] [45000]: (conn=10) Unable to insert an overlapping reservation
By the way, I think you have an error in your table definition. Instead of CHECK(Start_Date >= End_Date), I think you meant CHECK(Start_Date <= End_Date),
Let me know if it helps.
Note : I did this on MariaDB but you can apply the same for any SQL DB.

Answer 3

This works in sql server. I don't have access at the moment to check against postgres. You will need to run in two different batches after creating your table
First
CREATE FUNCTION dbo.HasOverlap (
#locationId int, #start datetime, #end datetime)
RETURNS VARCHAR(5)
AS
BEGIN
IF (SELECT count(*) FROM dbo.Reservation WHERE Location = #locationId
and (
#start between Start_Date and End_Date
or
#end between Start_Date and End_Date
or
(#start <=Start_Date and #end>=End_Date )
)
) >1
return 1
return 0
END
Second
Alter Table dbo.Reservation
with check add Constraint Check_Overlap
check (dbo.HasOverlap(Location, Start_Date, End_Date)=0)

Answer 4

You need to use the composite primary key concept in MySQL Database. Its disable to insert duplicate items in specific columns.
ALTER TABLE table_name ADD CONSTRAINT constraint_name PRIMARY KEY (Start_Date, End_Date, Location)