I was thinking the NATURAL JOIN
example you just used
SELECT * FROM my_table NATURAL JOIN (
SELECT group_col, MAX(sort_col) sort_col
FROM my_table
GROUP BY group_col
) t
If you shift to another type of JOIN
and impose WHERE
, ordering can come and go without warning in spite of the ill-advised reliance on undocumented behavior of the GROUP BY
.
For this example, I will
- use Windows 7
- use MySQL 5.5.12-log for Windows
- create some sample data
- impose a
LEFT JOIN
without a WHERE
clause
- impose a
LEFT JOIN
with a WHERE clause
For the DB Environment
mysql> select version();
+------------+
| version() |
+------------+
| 5.5.12-log |
+------------+
1 row in set (0.00 sec)
mysql> show variables like '%version_co%';
+-------------------------+------------------------------+
| Variable_name | Value |
+-------------------------+------------------------------+
| version_comment | MySQL Community Server (GPL) |
| version_compile_machine | x86 |
| version_compile_os | Win64 |
+-------------------------+------------------------------+
3 rows in set (0.00 sec)
mysql>
Using this script to generate sample data
DROP DATABASE IF EXISTS eggyal;
CREATE DATABASE eggyal;
USE eggyal
CREATE TABLE groupby
(
id int not null auto_increment,
num int,
primary key (id)
);
INSERT INTO groupby (num) VALUES
(floor(rand() * unix_timestamp())),(floor(rand() * unix_timestamp())),
(floor(rand() * unix_timestamp())),(floor(rand() * unix_timestamp())),
(floor(rand() * unix_timestamp())),(floor(rand() * unix_timestamp())),
(floor(rand() * unix_timestamp())),(floor(rand() * unix_timestamp()));
INSERT INTO groupby (num) SELECT num FROM groupby;
SELECT * FROM groupby;
and these two queries for testing the GROUP BY
subsequent use;
SELECT * FROM groupby A LEFT JOIN
(
SELECT num, MAX(id) id
FROM groupby
GROUP BY num
) B USING (id);
SELECT * FROM groupby A LEFT JOIN
(
SELECT num, MAX(id) id
FROM groupby
GROUP BY num
) B USING (id) WHERE B.num IS NOT NULL;
Let's test the durability of the GROUP BY
's results;
STEP 01 : Create the Sample Data
mysql> DROP DATABASE IF EXISTS eggyal;
Query OK, 1 row affected (0.09 sec)
mysql> CREATE DATABASE eggyal;
Query OK, 1 row affected (0.00 sec)
mysql> USE eggyal
Database changed
mysql> CREATE TABLE groupby
-> (
-> id int not null auto_increment,
-> num int,
-> primary key (id)
-> );
Query OK, 0 rows affected (0.07 sec)
mysql> INSERT INTO groupby (num) VALUES
-> (floor(rand() * unix_timestamp())),(floor(rand() * unix_timestamp())),
-> (floor(rand() * unix_timestamp())),(floor(rand() * unix_timestamp())),
-> (floor(rand() * unix_timestamp())),(floor(rand() * unix_timestamp())),
-> (floor(rand() * unix_timestamp())),(floor(rand() * unix_timestamp()));
Query OK, 8 rows affected (0.06 sec)
Records: 8 Duplicates: 0 Warnings: 0
mysql> INSERT INTO groupby (num) SELECT num FROM groupby;
Query OK, 8 rows affected (0.05 sec)
Records: 8 Duplicates: 0 Warnings: 0
mysql> SELECT * FROM groupby;
+----+------------+
| id | num |
+----+------------+
| 1 | 269529129 |
| 2 | 387090406 |
| 3 | 1126864683 |
| 4 | 411160755 |
| 5 | 29173595 |
| 6 | 266349579 |
| 7 | 1244227156 |
| 8 | 6231766 |
| 9 | 269529129 |
| 10 | 387090406 |
| 11 | 1126864683 |
| 12 | 411160755 |
| 13 | 29173595 |
| 14 | 266349579 |
| 15 | 1244227156 |
| 16 | 6231766 |
+----+------------+
16 rows in set (0.00 sec)
STEP 02 : Use LEFT JOIN
without a WHERE
clause
mysql> SELECT * FROM groupby A LEFT JOIN
-> (
-> SELECT num, MAX(id) id
-> FROM groupby
-> GROUP BY num
-> ) B USING (id);
+----+------------+------------+
| id | num | num |
+----+------------+------------+
| 1 | 269529129 | NULL |
| 2 | 387090406 | NULL |
| 3 | 1126864683 | NULL |
| 4 | 411160755 | NULL |
| 5 | 29173595 | NULL |
| 6 | 266349579 | NULL |
| 7 | 1244227156 | NULL |
| 8 | 6231766 | NULL |
| 9 | 269529129 | 269529129 |
| 10 | 387090406 | 387090406 |
| 11 | 1126864683 | 1126864683 |
| 12 | 411160755 | 411160755 |
| 13 | 29173595 | 29173595 |
| 14 | 266349579 | 266349579 |
| 15 | 1244227156 | 1244227156 |
| 16 | 6231766 | 6231766 |
+----+------------+------------+
16 rows in set (0.00 sec)
mysql>
STEP 03 : Use LEFT JOIN
with a WHERE
clause
mysql> SELECT * FROM groupby A LEFT JOIN
-> (
-> SELECT num, MAX(id) id
-> FROM groupby
-> GROUP BY num
-> ) B USING (id) WHERE B.num IS NOT NULL;
+----+------------+------------+
| id | num | num |
+----+------------+------------+
| 16 | 6231766 | 6231766 |
| 13 | 29173595 | 29173595 |
| 14 | 266349579 | 266349579 |
| 9 | 269529129 | 269529129 |
| 10 | 387090406 | 387090406 |
| 12 | 411160755 | 411160755 |
| 11 | 1126864683 | 1126864683 |
| 15 | 1244227156 | 1244227156 |
+----+------------+------------+
8 rows in set (0.00 sec)
mysql>
ANALYSIS
Looking at the aforementioned results, here are two questions:
- Why does a
LEFT JOIN
keep an ordering by id
?
- Why in the world did using a
WHERE
impose a reordering ?
- Was it during the JOIN phase ?
- Did the Query Optimizer look ahead at the ordering of the subquery or ignore it ?
No one foresaw any of these effects because the behavior of explicit clauses was relied upon by the implicit behavior of the Query Optimizer.
CONCLUSION
From my perspective, corner cases can only be of an external nature. In light of this, developers must be willing to fully evaluate the results of a GROUP BY
in conjunction with the following twelve(12) aspects:
- aggregate functions
- subquery usage
JOINs
clauses
WHERE
clauses
- sort order of results with no explicit
ORDER BY
clause
- query results using older GA releases of MySQL
- query results using newer beta releases of MySQL
- the current SQL_MODE setting in
my.cnf
- the operating system the code was compiled for
- possibly the size of join_buffer_size with respect to its effect on the Query Optimizer
- possibly the size of sort_buffer_size with respect to its effect on the Query Optimizer
- possibly the storage engine being used (MyISAM vs InnoDB)
Here is the key thing to remember : Any instance of MySQL that works for your query in a specific environment is itself a corner case. Once you change one or more of the twelve(12) evaluation aspects, the corner case is due to break, especially given the first nine(9) aspects.
The traditional solution, the one you may find in books, is to do a self join: first find that "max date per group" of yours, then join to self table on rows with said max date.
However, some hacks allow you to avoid that. Consider the following query:
SELECT
MAX(date_added) AS date_added,
SUBSTRING_INDEX(GROUP_CONCAT(some_column ORDER BY date_added DESC), ',', 1) AS some_column,
SUBSTRING_INDEX(GROUP_CONCAT(another_column ORDER BY date_added DESC), ',', 1) AS another_column
FROM t
GROUP BY whatever
GROUP_CONCAT
is an aggregation function which implodes values onto one string. It allows for ORDER BY
, which we utilize via ORDER BY date_added DESC
so as to implode our desired value first. We then slice up the first token in the string via SUBSTRING_INDEX
.
The downside here (apart from making the query quite the frightening appearance) is that your numerical values are transformed into texts. Typically no big deal with SQL, but please be aware.
See also my old post: Selecting a specific non aggregated column data in GROUP BY
There's another option where you do a semi-self-join, a much lighter one; you will have to give up usage of index. It's quite long to describe; it still uses GROUP_CONCAT
and SUBSTRING_INDEX
, but only for the purpose of creating a derived table with only relevant keys. This derived table is then joined with original table. See an example in SQL: selecting top N records per group, another solution.
Best Answer
As an aggregate function the BIT_OR is evaluated as a bitwise OR of all the
value
s in the group. If it equates to 0, none of the value have been set.