is_unique syntax in mysql? - php

Question

I have a table entity called position. I am storing the value using smallint(2) datatype, where I intend to store the value from 0 to 9 (max). I want to make sure the value is just a single integer and an integer should not be repeated at any cost. There should be a unique value assigned to each id of the position entity in the table ranging from 0 to 9. How do I achieve it? Is there any mysql syntax for that?
I am using MySQL with PHP.
here is the mysql table could someone add me the unique code and show
CREATE TABLE `advertisements` (
`id` int(11) NOT NULL auto_increment,
`title` varchar(255) NOT NULL,
`position` smallint(2) NOT NULL,
`active` tinyint(1) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
is this the correct syntax?
CREATE TABLE `advertisements` (
`id` int(11) NOT NULL auto_increment,
`title` varchar(255) NOT NULL,
`position` smallint(2) NOT NULL UNIQUE,
`active` tinyint(1) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Answer 1

You must define that column as UNIQUE.
ALTER TABLE `a` ADD UNIQUE (
`id`
)

Answer 2

Your question has two parts:
How do I ensure that a value is unique in a column?
Add a unique index.
How do I restrict a value to be in the range 0 to 9?
You can add a foreign key constraint to a table that lists the allowed values.

Answer 3

First of all, smallint(2) and smallint(1234) and smallint are exactly the same in MySQL. The numeric argument is not a limit on the length or the range of values, it's only a hint for display width. MySQL's smallint is a 16-bit integer, with a range from -32768 to 32767. smallint unsigned has a range from 0 to 65535.
You can ensure no duplicates exist in a column by declaring a PRIMARY KEY or a UNIQUE KEY:
CREATE TABLE MyTable (
position SMALLINT NOT NULL,
UNIQUE KEY (position)
);
To restrict the values to the range 0 to 9, there are a few options:
Write a trigger that causes an error if you try to store a value outside the range.
DELIMITER //
CREATE TRIGGER ZeroToNine BEFORE INSERT ON MyTable
FOR EACH ROW
BEGIN
DECLARE p SMALLINT;
IF (NEW.position NOT BETWEEN 0 AND 9) THEN
SET p = 'Position must be between 0 and 9';
END IF;
END //
DELIMITER ;
Use a foreign key constraint so the column references a table that you populate with the values 0 to 9.
CREATE TABLE ZeroToNine (
position SMALLINT NOT NULL PRIMARY KEY
) ENGINE=InnoDB;
INSERT INTO ZeroToNine VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9);
CREATE TABLE MyTable (
position SMALLINT NOT NULL,
UNIQUE KEY (position),
FOREIGN KEY (position) REFERENCES ZeroToNine (position)
) ENGINE=InnoDB;
Other databases also support a CHECK constraint, but MySQL unfortunately doesn't support this feature.
CREATE TABLE MyTable (
position SMALLINT NOT NULL
CHECK (position BETWEEN 0 AND 9),
UNIQUE KEY (position)
);