防止使用PHP将相同的名称插入MySQL

Question

How can i prevent inserting a value into database if it is already exist with the same name in it? echoing alert that this name already exist for example DATABASE has [MARY, MIKE, JOHN] and then someone tried to INSERT the name [JOHN] again, I want to prevent that from happening, Even if he changed the letter style [JoHn]

Answer 1

Add a UNIQUE index on the column(s) and make sure the column has a case-insensitive (preferably accent-insensitive too) collation. That should do it, I think.

If you try to SELECT first and see if the name is there, you can run afoul of a "race condition". Basically there's a small amount of time between you running the SELECT and INSERT, and someone else can manage to INSERT the name during that. It's a small chance, but it's there, and eventually it will bite you. You can counteract this with a transaction (be careful about the isolation level your transaction uses), but why bother when an index is so much simpler and easier? Also, you'll need an index on that column anyway if you want your check to be fast.

Answer 2

When defining a table UNIQUE may be specified for a given column so a duplicate entry attempt will result in an error (the INSERT query will fail).

For example:

CREATE TABLE `test` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL UNIQUE,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=UTF8;

Normally default collation for a varchar is case insensitive (checking would not be bad however).

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Another option is to first execute a SELECT query to find if a record with the same field value already exists and then perform a second INSERT query if no rows are found or display an error otherwise

Answer 3

You could also use a trigger:

DELIMITER $$
CREATE TRIGGER if_exists BEFORE INSERT ON table_name
FOR EACH ROW
BEGIN
    IF EXISTS(SELECT name FROM table_name WHERE LOWER(name) = LOWER(NEW.name)) THEN
        SIGNAL sqlstate '45000' SET message_text = 'Error! User already exists.';
    END IF;
END$$
DELIMITER ;

Replace table_name with the name of your table and name with the name of the attribute you want to compare. Signals are supported by MySQL 5.5 and above.

Answer 4

插入之前，从表中选择用户名LIKE用户名