CoolData blog

5 December 2016

Amazing things with matching strings

Filed under: Coolness, Data integrity, SQL — Tags: , , , — kevinmacdonell @ 7:44 am

 

I had an occasion recently when it would have been really helpful to know that a new address added to the database was a duplicate of an older, inactivated address. The addition wasn’t identified as a duplicate because it wasn’t a perfect match — a difference similar to that between 13 Anywhere Road and 13 Anywhere Drive. 

 

After the fact, I did a Google search and discovered some easy-to-use functionality in Oracle SQL that might have saved us some trouble. Today I want to talk about how to use UTL_MATCH and  suggest some cool applications for it in Advancement data work.

 

“Fuzzy matching” is the term used for identifying pairs of character strings that may not be exactly the same, but are so close that they could be. For example, “Washignton” is one small typo away from “Washington,” but the equivalence is very difficult to detect by any means other than an alert pair of human eyes scanning a sorted list. When the variation occurs at the beginning of a string — “Unit 3, 13 Elm St.” instead of “Apmt 3, 13 Elm St.” — then even a sorted list is of no use.

 

According to this page, the UTL_MATCH package was introduced in Oracle 10g Release 2, but first documented and supported in Oracle 11g Release 2. The package includes two functions for testing the level of similarity or difference between strings.

 

The first function is called EDIT_DISTANCE, which is a count of the number of “edits” to get from one string to a second string. For example, the edit distance from “Kevin” to “Kelvin” is 1, for “New York” to “new york” is 2, and from “Hello” to “Hello” is 0. (A related function, EDIT_DISTANCE_SIMILARITY, expresses the distance as a normalized value between 0 and 100 — 100 being a perfect match.)

 

The second method, the one I’ve been experimenting with, is called JARO_WINKLER, named for an algorithm that measures the degree of similarity between two strings. The result ranges between 0 (no similarity) to 1 (perfect similarity). It was designed specifically for detecting duplicate records, and its formula seems aimed at the kind of character transpositions you’d expect to encounter in data entry errors. (More info here: Jaro-Winkler distance.)

 

Like EDIT_DISTANCE, it has a related function called JARO_WINKLER_SIMILARITY. Again, this ranges from 0 (no match) to 100 (perfect match). This is the function I will refer to for the rest of this post.

 

Here is a simple example of UTL_MATCH in action. The following SQL scores constituents in your database according to how similar their first name is to their last name, with the results sorted in descending order by degree of similarity. (Obviously, you’ll need to replace “schema”, “persons,” and field names with the proper references from your own database.)

 

SELECT

t1.ID,

t1.first_name,

t1.last_name,

UTL_MATCH.jaro_winkler_similarity(t1.first_name, t1.last_name) AS jw

FROM schema.persons t1

ORDER BY jw DESC

 

Someone named “Donald MacDonald” would get a fairly high value for JW, while “Kevin MacDonell” would score much lower. “Thomas Thomas” would score a perfect 100.

 

Let’s turn to a more useful case: Finding potential duplicate persons in your database. This entails comparing a person’s full name with the full name of everyone else in the database. To do that, you’ll need a self-join.

 

In the example below, I join the “persons” table to itself. I concatenate first_name and last_name to make a single string for the purpose of matching. In the join conditions, I exclude records that have the same ID, and select records that are a close or perfect match (according to Jaro-Winkler). To do this, I set the match level at some arbitrary high level, in this case greater than or equal to 98.

 

SELECT

t1.ID,

t1.first_name,

t1.last_name,

t2.ID,

t2.first_name,

t2.last_name,

UTL_MATCH.jaro_winkler_similarity ( t1.first_name || ' ' || t1.last_name, t2.first_name || ' ' || t2.last_name ) AS jw

FROM schema.persons t1

INNER JOIN schema.persons t2 ON t1.ID != t2.ID AND UTL_MATCH.jaro_winkler_similarity ( t1.first_name || ' ' || t1.last_name, t2.first_name || ' ' || t2.last_name ) >= 98

ORDER BY jw DESC

 

I would suggest reading this entire post before trying to implement the example above! UTL_MATCH presents some practical issues which limit what you can do. But before I share the bad news, here are some exciting possible Advancement-related applications:

 

  • Detecting duplicate records via address matching.
  • Matching external name lists against your database. (Which would require the external data be loaded into a temporary table in your data warehouse, perhaps.)
  • Screening current and incoming students against prospect, donor, and alumni records for likely matches (on address primarily, then perhaps also last name).
  • Data integrity audits. An example: If the postal code or ZIP is the same, but the city name is similar (but not perfectly similar), then there may be an error in the spelling or capitalization of the city name.
  • Searches on a particular name. If the user isn’t sure about spelling, this might be one way to get suggestions back that are similar to the guessed spelling.

 

Now back to reality … When you run the two code examples above, you will probably find that the first executes relatively quickly, while the second takes a very long time or fails to execute at all. That is due to the fact that you’re evaluating each record in the database against every other record. This is what’s known as a cross-join or Cartesian product — a very costly join which is rarely used. If you try to search for matches across 100,000 records, that’s 10 billion evaluations! The length of the strings themselves contributes to the complexity, and therefore the runtime, of each evaluation — but the real issue is the 10,000,000,000 operations.

 

As intriguing as UTL_MATCH is, then, its usage will cause performance issues. I am still in the early days of playing with this, but here are a few things I’ve learned about avoiding problems while using UTL_MATCH.

 

Limit matching records. Trying to compare the entire database with itself is going to get you in trouble. Limit the number of records retrieved for comparison. A query searching for duplicates might focus solely on the records that have been added or modified in the past day or two, for example. Even so, those few records have to be checked against all existing records, so it’s still a big job — consider not checking against records that are marked deceased, that are non-person entities, and so on. Anything to cut down on the number of evaluations the database has to perform.

 

Keep strings short. Matching works best when working with short strings. Give some thought to what you really want to match on. When comparing address records, it might make sense to limit the comparison to Street Line 1 only, not an entire address string which could be quite lengthy.

 

Pre-screen for perfect matches: A Jaro-Winkler similarity of 100 means that two strings are exactly equal. I haven’t tested this, but I’m guessing that checking for A = B is a lot faster than calculating the JW similarity between A and B. It might make sense to have one query to audit for perfect matches (without the use of UTL_MATCH) and exclude those records from a second query that audits for JW similarities that are high but less than a perfect 100.

 

Pre-screen for impossible matches. If a given ID_1 has a street address than is 60 characters long and a given ID_2 has a street address that is only 20 characters long, there is no possibility of a high Jaro-Winkler score and therefore no need to calculate it. Find a way to limit the data set to match before invoking UTL_MATCH, possibly through the use of a WITH clause that limits potential matching pairs by excluding any that differ in length by more than, say, five characters. (Another “pre-match” to use would check if the initial letter in a name is the same; if it isn’t, good chance it isn’t going to be a match.)

 

Keep match queries simple. Don’t ask for fields other than ID and the fields you’re trying to match on. Yes, it does make sense to bring down birthdate and additional address information so that the user can decide if a probable match is a true duplicate or not, but keep that part of the query separate from the match itself. You can do this by putting the match in a WITH clause, and then left-joining additional data to the results of that clause.

 

Truth be told, I have not yet written a query that does something useful while still executing in a reasonable amount of time, simply due to the sheer number of comparisons being made. I haven’t given up on SQL, but it could be that duplicate detection is better accomplished via a purpose-built script running on a standalone computer that is not making demands on an overburdened database or warehouse (aside from the initial pull of raw data for analysis).

 

The best I’ve done so far is a query that selects address records that were recently modified and matches them against other records in the database. Before it applies Jaro-Winkler, the query severely limits the data by pairing up IDs that have name strings and address strings that are nearly the same number of characters long. The query has generated a few records to investigate and, if necessary, de-dupe — but it takes more than an hour and half to run.

 

Have any additional tips for making use of UTL_MATCH? I’d love to hear and share. Email me at kevin.macdonell@gmail.com.

 

Advertisements

Create a free website or blog at WordPress.com.

%d bloggers like this: