Searching on Two Keys
220
*************************** 1. row ***************************
Table: shirt
Create Table: CREATE TABLE `shirt` (
`id` smallint(5) unsigned NOT NULL auto_increment,
`style` enum('t-shirt','polo','dress') NOT NULL,
`color` enum('red','blue','orange','white','black') NOT NULL,
`owner` smallint(5) unsigned NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1
The use of
REFERENCES
in this way as a comment or “reminder” in a column definition works with both
MyISAM
and
BerkeleyDB
tables.
3.6.7. Searching on Two Keys
An
OR
[881]
using a single key is well optimized, as is the handling of
AND
[880]
.
The one tricky case is that of searching on two different keys combined with
OR
[881]
:
SELECT field1_index, field2_index FROM test_table
WHERE field1_index = '1' OR field2_index = '1'
This case is optimized from MySQL 5.0.0. See
Section 8.3.1.4, “Index Merge Optimization”
.
You can also solve the problem efficiently by using a
UNION
that combines the output of two separate
SELECT
statements. See
Section 13.2.8.4, “
UNION
Syntax”
.
Each
SELECT
searches only one key and can be optimized:
SELECT field1_index, field2_index
FROM test_table WHERE field1_index = '1'
UNION
SELECT field1_index, field2_index
FROM test_table WHERE field2_index = '1';
3.6.8. Calculating Visits Per Day
The following example shows how you can use the bit group functions to calculate the number of days
per month a user has visited a Web page.
CREATE TABLE t1 (year YEAR(4), month INT(2) UNSIGNED ZEROFILL,
day INT(2) UNSIGNED ZEROFILL);
INSERT INTO t1 VALUES(2000,1,1),(2000,1,20),(2000,1,30),(2000,2,2),
(2000,2,23),(2000,2,23);
The example table contains year-month-day values representing visits by users to the page. To
determine how many different days in each month these visits occur, use this query:
SELECT year,month,BIT_COUNT(BIT_OR(1<<day)) AS days FROM t1
GROUP BY year,month;
Which returns:
+------+-------+------+
| year | month | days |
+------+-------+------+
| 2000 | 01 | 3 |
| 2000 | 02 | 2 |
+------+-------+------+
The query calculates how many different days appear in the table for each year/month combination,
with automatic removal of duplicate entries.
3.6.9. Using
AUTO_INCREMENT
The
AUTO_INCREMENT
attribute can be used to generate a unique identity for new rows:
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