Mysql - Preventing duplicate entries of combined columns with Unique Index - mysql

Question

I have an issue with duplicate entries in a database and due to the nature of the problem the easiest way to fix it would be to remove current duplicate rows and prevent further duplicates from being added .
Here is the table structure :
| a | b | c |
user url1 token1
photo url1 token2
action action1 token3
user url1 token4
photo url1 token5
action action2 token6
I want to prevent duplicate entries only when 2 columns are duplicated, in this case a and b .
So here we have user | url1 and photo | url1 duplicated twice.
I want to prevent any further duplicates from being added when both columns match another row at same time but the queries I found so far will consider each column separately and preventing any further duplicates to be added to any of them .
Can I achieve this with a mysql query using unique index ?
I tried using the following code :
Using ALTER TABLE `targets` ADD UNIQUE (
`a` ,
`b`
);

Answer 1

Your question:"Can I achieve this with a mysql query using unique index ?"
Answer is 100% yes.
There are two ways of creating index:
1. CREATE UNIQUE INDEX index_name
ON table_name (column1, column2, ...);
2. ALTER TABLE table_name
ADD UNIQUE index_name (column1, column2, ...);
However, this will only work if your table doesn't have existing duplicate data. Otherwise you'll receive an error message like this:
Query: CREATE UNIQUE INDEX index_name ON targets (a, b)
Error Code: 1062
Duplicate entry 'photo-url1' for key 'index_name'
Therefore, you need to:
create a new empty table similar to your targets table.
create unique index.
INSERT IGNORE data from the old table.
Rename targets to targets_old and targets_new to targets.
Example:
CREATE TABLE targets_new LIKE targets;
CREATE UNIQUE INDEX index_name
ON targets_new (a, b);
INSERT IGNORE INTO targets_new SELECT * FROM targets;
RENAME TABLE targets TO targets_old;
RENAME TABLE targets_new TO targets;

Answer 2

Thanks for the replies guys, but I found the solution in the meantime and it was much simpler !
It's called unique composite key and it allows to do exactly what I wanted :
ALTER TABLE targets ADD UNIQUE KEY `uidx` (a, b, c);
Problem fixed :)

Answer 3

i don't think you can specify a unique doublet property.
edit : mention by Salmon : An index specification of the form (key_part1, key_part2, ...) creates an index with multiple key parts
https://dev.mysql.com/doc/refman/8.0/en/create-index.html#create-index-unique
you could try :
SELECT * FROM database1.table1 WHERE a='user' AND b='url1';
if got return a number of rows then don't add to the database.
this way you can fully control what goes into the database instead of letting the table automatically ignore the fail condition.