14

How can I check whether a field from a table is set as UNIQUE?

For example I have a table named users with a field email set as UNIQUE and a field picture not set as UNIQUE, I want before selecting check whether the field is set set as UNIQUE if not then don't do the SELECT.

I tried to SELECT then count the returned number of row, if more than 1 then it's not UNIQUE, 

"SELECT * FROM table WHERE email='$email'"
//...some mysql php line later
if($count > 1){
    //return nothing
}

but it's not efficient, what if there is no duplicate.

What's the best way to check whether a field is set as UNIQUE in PHP?

Edit: no duplicate doesn't mean it has UNIQUE property

Arius
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Jonathan de M.
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  • alternatively you can check via `phpmyadmin` – xkeshav Dec 27 '12 at 04:38
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    Your code should already know whether a field has a UNIQUE constraint on it; anything else is just reverse engineering your own application. – Ja͢ck Dec 27 '12 at 05:01
  • Basically I have a function to search by and the result should be unique (kinda like a slug), like `username=bob` `email=boby@dgf.fg` but if the username is not unique then the user could not do a search by username and I'd throw an error. – Jonathan de M. Sep 11 '13 at 12:19

3 Answers3

16

From the documentation of SHOW INDEX (found by @diEcho):

SHOW INDEX returns the following fields:

Non_unique -- 0 if the index cannot contain duplicates, 1 if it can.

Column_name -- The column name.

Try:

SHOW INDEXES
FROM $tablename
WHERE Column_name='$field'
AND NOT Non_unique

Note that this assumes that there is no UNIQUE index that spans multiple columns. If there can be, then you might want to exclude these with a subquery.

Also note disabled indexes also show in this query (the possibility of disabled indexes is mentioned in the documentation on the Comment column). There doesn't seem to be a column reflecting this, so you might need to parse the Comment column if you have disabled indexes.

There's no need to compare Non_unique to a number - MySQL uses 0 and 1 for booleans anyways

Community
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John Dvorak
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2

Ok I found it thanks to @diEcho

public function isUniqueField($tablename, $field, $connection){
        $query = $connection->query("SHOW INDEXES FROM $tablename WHERE Column_name='$field' AND Non_unique=0");
        $query->execute();
        if(!$query->fetchAll()){
            return false;
        }
        return true;
    }
Jonathan de M.
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  • Just because there's an index doesn't mean there's a unique index. You need to check that as well. – John Dvorak Dec 27 '12 at 05:00
  • `SHOW INDEX returns the following fields:` `Non_unique -- 0 if the index cannot contain duplicates, 1 if it can.` `Key_name -- The name of the index. If the index is the primary key, the name is always PRIMARY.` `Column_name -- The column name.` – John Dvorak Dec 27 '12 at 05:03
1

You can check all indexed column with

SHOW INDEX

if there is a UNIQUE index then it cant be duplicate value in same table BUT a UNIQUE index allows multiple NULL values for columns that can contain NULL

update

to create a UNIQUE constraint on a column(let email) use below query

  ALTER TABLE  `table_name` ADD UNIQUE (`email`);
xkeshav
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