CODEWITHSUNDEEP
sqlsqlitedistinct
Tony Vitabile
Tony Vitabile

Reputation: 8594

Returning distinct rows using WHERE Exists(SELECT 1 FROM TABLE)

I've got two tables in my SQLite database with a one to many relationship between them:

CREATE TABLE Reads (
    RowID     INTEGER NOT NULL PRIMARY KEY,
    EventDate INTEGER NOT NULL,
    Plate     TEXT,
    State     TEXT
);

CREATE TABLE ExplodedPlates (
    Plate     TEXT,
    RowID     INTEGER NOT NULL,
    PRIMARY KEY (RowID, Plate)
) WITHOUT RowId;

The Reads table's Plate column contains license plates. The data may contain what we call "ambiguous license plates". For example, a plate may contain "A[0OQ]C1234", where the actual second character could be a "0", an "O", or a "Q". In this case, there are three rows in the ExplodedPlates table with the same RowID but the Plate is "A0C123", another with "AOC1234", and a third with "AQC123".

I need to join these tables and return one row for each unique RowID where the plate matches a pattern similar to the one in the example. So if the user enters "A[O0]1234" in the search box, they should get one row with the plate "A[0OQ]1234", but not 3, as well as any rows with other RowIDs that match that string.

I've written a query using Entity Framework similar to this one:

SELECT DISTINCT r.*
FROM ExplodedPlates A x
JOIN Reads AS r ON r.RowId = x.RowID
WHERE x.Plate GLOB @Plate

This works, but the query returned uses a temporary B-Tree to do the DISTINCT. My boss wants me to get rid of the DISTINCT and use WHERE EXISTS(SELECT 1 FROM ExplodedPlates WHERE Plate GLOB @Plate), but I don't know how to get this to work. I have a specific case in my database with 135 rows that match a particular pattern, but the query I'm using returns 6557 rows. Clearly, this is wrong.

How exactly can I use that WHERE clause to generate the unique list of matching reads?

Upvotes: 0

Views: 1639

Answers (1)

Gordon Linoff
Gordon Linoff

Reputation: 1269443

This may be what you want:

select r.*
from reads r
where exists (select 1
              from ExplodedPlates x
              where r.RowId = x.RowID and
                    x.Plate GLOB @Plate
             );

Upvotes: 1

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