Operating within database software can sometimes feel like navigating a minefield. The vast expanse of data sets, tables and functions can leave a user tip-toeing throughout, careful not to enter a mistaken query or improper syntax that could halt the data in an instant. A software as user-friendly and accessible as Oracle can even occasionally feel problematic, perhaps seeming most daunting when writing statements for it to process. The ORA-01427 error can be the manifestation of these fears. Fortunately, the error has a couple of basic approaches that will remedy the situation and have your code back up and running in no time.
The ORA-01427 is a straightforward query error in Oracle. The cause of the error is a subquery returning more than one row of information. This error in multi-row returns on the subquery originates from an outer query failing to use appropriate, designated keywords to specify values for comparison in the subquery.
Before we continue with solving this error, let us briefly discuss subqueries in Oracle. The definition of an Oracle subquery is fairly tautological: a subquery is a query within a query. To review, a query is simply a function used to retrieve specific data from the tables within a database. The query allows a user to view data spread across a database on one single datasheet. A subquery furthers this process, allowing the user to issue a query within the SQL statement to refine a request.
A subquery can be placed in a WHERE clause, a FROM clause, or a SELECT clause depending on its purpose. Most often the subquery will be proposed in the WHERE clause (also known as a “nested subquery”). Oracle will allow a user to issue subqueries in the WHERE clause up to 255 levels. A subquery can also be in the FROM clause (a “inline subquery”) or a SELECT clause, however a subquery placed in the SELECT clause must return a single value.
One necessity of the subquery is that it returns just one row or otherwise includes certain keywords in the outer query. The keywords are ANY, ALL, IN or NOT IN. These will aid Oracle in specifying values to compare, creating a concise return.
This brings us to our solution for the ORA-01427 error. There are essentially two options to take. The first is to rewrite the query so that the subquery will only return a single row, thus eliminating the source of the error. This will require a user to rethink how they wish to acquire the data that they were initially searching for. The alternative option that will allow the user to more quickly assume where they left off is to change the query by inserting one of the previously mentioned keywords (ANY, ALL, IN or NOT IN). Let us look at how this would play out in a brief example.
Perhaps the user attempted to issue the following SQL statement:
WHERE client_id = (SELECT client_id
WHERE clients_name = ‘Walmart’) ;
If more than one instance in the client’s table of the clients_name ‘Walmart’ existed, the following would be prompted:
ORA-01427: single-row subquery returns more than one row
The user can then correct the statement by utilizing an IN condition, which is used to reduce the need for multiple OR conditions in SELECT statements:
WHERE client_id IN (SELECT client_id
WHERE clients_name = ‘Walmart’) ;
This will eliminate the ORA-01427 error message from occurring.
Perhaps what takes database managers the longest to master is coding SQL statements. It can be incredibly easy to make what seem like the simplest of mistakes after they are corrected. Oracle has taken a lot of steps in mitigating user frustration when coding, but errors can always arise. If you are finding that you are having a difficult time with coding SQL statements, or maybe want a little more advice on creating cleaner statements, feel free to contact a licensed Oracle consultant for more information.