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Asked: 2026-05-10T18:12:59+00:00

I’m getting odd results from a MySQL SELECT query involving a LEFT JOIN ,

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I’m getting odd results from a MySQL SELECT query involving a LEFT JOIN, and I can’t understand whether my understanding of LEFT JOIN is wrong or whether I’m seeing a genuinely odd behavior.

I have a two tables with a many-to-one relationship: For every record in table 1 there are 0 or more records in table 2. I want to select all the records in table 1 with a column that counts the number of related records in table 2. As I understand it, LEFT JOIN should always return all records on the LEFT side of the statement.

Here’s a test database that exhibits the problem:

CREATE DATABASE Test; USE Test;  CREATE TABLE Dates (    dateID INT UNSIGNED NOT NULL AUTO_INCREMENT,    date DATE NOT NULL,    UNIQUE KEY (dateID) ) TYPE=MyISAM;   CREATE TABLE Slots (    slotID INT UNSIGNED NOT NULL AUTO_INCREMENT,    dateID INT UNSIGNED NOT NULL,    UNIQUE KEY (slotID) ) TYPE=MyISAM;  INSERT INTO Dates (date) VALUES ('2008-10-12'),('2008-10-13'),('2008-10-14'); INSERT INTO Slots (dateID) VALUES (3);

The Dates table has three records, and the Slots 1 – and that record points to the third record in Dates.

If I do the following query..

SELECT d.date, count(s.slotID) FROM Dates AS d LEFT JOIN Slots AS s ON s.dateID=d.dateID GROUP BY s.dateID;

..I expect to see a table with 3 rows in – two with a count of 0, and one with a count of 1. But what I actually see is this:

+------------+-----------------+ | date       | count(s.slotID) | +------------+-----------------+ | 2008-10-12 |               0 | | 2008-10-14 |               1 | +------------+-----------------+

The first record with a zero count appears, but the later record with a zero count is ignored.

Am I doing something wrong, or do I just not understand what LEFT JOIN is supposed to do?

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1 Answer

  1. You need to GROUP BY d.dateID. In two of your cases, s.DateID is NULL (LEFT JOIN) and these are combined together.

    I think you will also find that this is invalid (ANSI) SQL, because d.date is not part of a GROUP BY or the result of an aggregate operation, and should not be able to be SELECTed.

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