Enforce unique rows in MySQL

Question

I have a table in MySQL that has 3 fields and I want to enforce uniqueness among two of the fields. Here is the table DDL:

CREATE TABLE `CLIENT_NAMES` (
`ID` int(11) NOT NULL auto_increment,
`CLIENT_NAME` varchar(500) NOT NULL,
`OWNER_ID` int(11) NOT NULL,
PRIMARY KEY  (`ID`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

The ID field is a surrogate key (this table is being loaded with ETL). The CLIENT_NAME is a field that contains names of clients The OWNER_ID is an id indicates a clients owner.

I thought I could enforce this with a unique index on CLIENT_NAME and OWNER_ID,

ALTER TABLE `DW`.`CLIENT_NAMES` 
ADD UNIQUE INDEX enforce_unique_idx(`CLIENT_NAME`, `OWNER_ID`);

but MySQL gives me an error:

Error executing SQL commands to update table. Specified key was too long; max key length is 765 bytes (error 1071)

Anyone else have any ideas?

Answer 1

MySQL cannot enforce uniqueness on keys that are longer than 765 bytes (and apparently 500 UTF8 characters can surpass this limit).

1. Does CLIENT_NAME really need to be 500 characters long? Seems a bit excessive.
2. Add a new (shorter) column that is hash(CLIENT_NAME). Get MySQL to enforce uniqueness on that hash instead.

Answer 2

Have you looked at CONSTRAINT ... UNIQUE?

Answer 3

Something seems a bit odd about this table; I would actually think about refactoring it. What do ID and OWNER_ID refer to, and what is the relationship between them?

Would it make sense to have

CREATE TABLE `CLIENTS` (
`ID` int(11) NOT NULL auto_increment,
`CLIENT_NAME` varchar(500) NOT NULL,
# other client fields - address, phone, whatever
PRIMARY KEY  (`ID`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `CLIENTS_OWNERS` (
`CLIENT_ID` int(11) NOT NULL,
`OWNER_ID` int(11) NOT NULL,
PRIMARY KEY  (`CLIENT_ID`,`OWNER_ID`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

I would really avoid adding a unique key like that on a 500 character string. It's much more efficient to enforce uniqueness on two ints, plus an id in a table should really refer to something that needs an id; in your version, the ID field seems to identify just the client/owner relationship, which really doesn't need a separate id, since it's just a mapping.

Answer 4

Here. For the UTF8 charset, MySQL may use up to 3 bytes per character. CLIENT_NAME is 3 x 500 = 1500 bytes. Shorten CLIENT_NAME to 250.

later: +1 to creating a hash of the name and using that as the key.