MySQL – Resolving Weird Results When Grouping and Self-Joining a Table

MySQL

I'm on MySQL 5.5.49, the results of the following query seem wrong to me.

Test data :

CREATE TABLE `test` (
  `invoice_item_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `invoice_id` int(10) unsigned NOT NULL,
  `previous_invoice_item_id` int(10) unsigned DEFAULT NULL,
  `price` decimal(12,4) unsigned NOT NULL,
  PRIMARY KEY (`invoice_item_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO test (invoice_id, previous_invoice_item_id, price)
VALUES (1, NULL, 1), (1, 1, 0);

Query :

SELECT
 ii.invoice_item_id,
 pii.invoice_item_id,
 ii.previous_invoice_item_id,
 ii.price - pii.price,
 ii.price,
 pii.price
FROM test ii
JOIN test pii ON pii.invoice_item_id = ii.previous_invoice_item_id
GROUP BY ii.invoice_item_id;

The problem is that ii.price - pii.price returns 0 when it should return -1. If I remove the GROUP BY clause, the result is correct. If the difference is positive, the result is also correct.

I'm at a loss to explain the result, can someone enlighten me?

Best Answer

It's because your price column is unsigned.

CASTing the values works ok:

SELECT
 ii.invoice_item_id,
 pii.invoice_item_id,
 ii.previous_invoice_item_id,
 cast(ii.price as signed) - cast(pii.price as signed ),
 ii.price,
 pii.price
FROM test ii
JOIN test pii ON pii.invoice_item_id = ii.previous_invoice_item_id
GROUP BY ii.invoice_item_id

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With the unique index you have on the table, the second subquery should be quite efficient. The first subquery would benefit from an index on (teacherid, startdate, groupid):

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The activity of altering big tables are done in phases:

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You need not hit information schema directly like this, try using SHOW TABLE STATUS from db like 'table_name'
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Edit : A simple python script to automate the process https://github.com/georgecj11/hotswap_mysql_table.git

Best Answer

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