UNIQUE constraint in MySQL rows - mysql

Question

I am performing an INSERT query in my database as such:
$query = "INSERT IGNORE INTO user VALUES ('', 0, $safe_email, '$hashed_password')";
$result = $db->query($query);
The 3rd row in the db table is email which I have set a unique constraint to.
If I try to insert a duplicate email, with the above query notice I have the INSERT IGNORE which won't insert the record if it is a duplicate to one that already exists in the db, however it won't give an error or any kind of indication that a duplicate record is trying be inserted.
I want to show a nice error message if a duplicate is found but with the INSERT IGNORE I am struggling to do this because it won't show an error it will just ignore the query.
So I need something like:
IF (duplicate entry found in db){
echo "User already exists";
}
END IF

Answer 1

Use normal insert query and implement the query in try-catch statement. 'Insert' query will fail if you try to insert same email since it is unique constraint. So you can catch the exception as the 'Insert' query fails.
Example:
try {
"your insert query";
} catch (Exception $e) {
"your insert failure exception"
}
NB: you can catch all the exceptions that occurred during the execution of insert query which will be more helpful

Answer 2

As per documentation on INSERT ... IGNORE:
If you use the IGNORE keyword, errors that occur while executing the INSERT statement are ignored. For example, without IGNORE, a row that duplicates an existing UNIQUE index or PRIMARY KEY value in the table causes a duplicate-key error and the statement is aborted. With IGNORE, the row still is not inserted, but no error occurs. Ignored errors may generate warnings instead, although duplicate-key errors do not.
You can issue a show warnings; or any compatible similar statement from your scripting language or SQL interface. If it returns one or more such warnings, may be one of them can be on such insertion issues. Using them, you can show proper error or consoling message to the end user.
Edit 1:
... but ... how do I throw my own error message instead of the default exception when using INSERT without the IGNORE.
You can define a BEFORE INSERT trigger to identify duplicate data row and throw custom error message when found one.
Example:
delimiter //
drop trigger if exists bi_table_trigger //
CREATE TRIGGER bi_table_trigger BEFORE INSERT ON table
FOR EACH ROW
BEGIN
declare rowCount int default 0;
declare error_message varchar(1024) default '';
SELECT COUNT(1) into rowCount FROM table
WHERE email = NEW.email;
IF ( rowCount > 0 ) THEN -- if( rowCount ) -- too works
set error_message =
concat( error_message, 'User with email \'' );
set error_message =
concat( error_message, NEW.email, '\' ' );
set error_message =
concat( error_message, 'already exists' );
-- throw the error
-- User with email '?' already exists ( example )
signal sqlstate 1062 set message_text = error_message;
END IF;
END;//
delimiter ;