I found a funny case in MySQL query SQL with prefix string match;
I created table like this;
CREATE TABLE `EpgInfo` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`title` varchar(100) NULL NOT NULL DEFAUL '',
PRIMARY KEY (`id`),
KEY `title` (`title`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
Then, I inserted 1,000,000 rows of data for random column title which length is less than 20.
I use 3 SQLs like this:
SQL #1: select * from EpgInfo2 where title like "快" limit 1;
SQL #2: select * from EpgInfo2 where title = "中" limit 1;
SQL #3: select * from EpgInfo2 where title like "中" limit 1;
And I found that:
SQL #1 costs 0.2s.
SQL #2 and SQL #3 cost 0.0s.
I used show status like %handl% to found the query plan, I found SQL #1 scan all index data, SQL #2 and SQL #3 did not.
Why?
Use EXPLAIN to see how MySQL handles the queries, it might give you a clue.
Also, try some other characters. Maybe MySQL is misinterpreting one of those as having a percent sign in it.
How "far" into the table is "快" vs "中"?
If "中" was the first row looked at then only one row would need to be looked at for the 2nd and 3rd query.
If the other was the last row then the entire table would need to be scanned.
Related
I need to find a record who dont have a specific value in CSV column. below is the table structure
CREATE TABLE `employee` (
`id` int NOT NULL AUTO_INCREMENT,
`first_name` varchar(100) NOT NULL,
`last_name` varchar(100) NOT NULL,
`keywords` text,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
Sample record1: 100, Sam, Thompson, "50,51,52,53"
Sample record2: 100, Wan, Thompson, "50,52,53"
Sample record3: 100, Kan, Thompson, "53,52,50"
50 = sports
51 = cricket
52 = soccer
53 = baseball
i need to find the employees name who has the tags of "sports,soccer,baseball" excluding cricket
so the result should return only 2nd and 3rd record in this example as they dont have 51(cricket) but all other 3 though in diff pattern.
My query is below, but i couldnt get it worked any more.
SELECT t.first_name,FROM `User` `t` WHERE (keywords like '50,52,53') LIMIT 10
is there anything like unlike option? i am confused how to get this worked.
You could use FIND_IN_SET:
SELECT t.first_name
FROM `User` `t`
WHERE FIND_IN_SET('50', `keywords`) > 0
AND FIND_IN_SET('52', `keywords`) > 0
AND FIND_IN_SET('53', `keywords`) > 0
AND FIND_IN_SET('51', `keywords`) = 0;
Keep in mind it could be slow. The correct way is to normalize your table structure.
FIND_IN_SET will do the job for you but it does not use indexes. This is not a bug it's a feature.
SUBSTRING_INDEX can use an index and return the data as you wish. You don't have an index on it at the moment, But the catch here is that TEXT fields cannot be fully indexed and what you have is a TEXT field.
Normalize!
This is what you really should be doing. It's not a good idea to store comma separated values in a database. You really should be having a keywords table and since the keywords will be short, you can have a char or varchar narrow column which can be fully indexed.
I have this table:
CREATE TABLE `page` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`sortorder` SMALLINT(5) UNSIGNED NOT NULL,
PRIMARY KEY (`id`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
;
This is the data I have:
id sortorder
1 0
2 1
And I want to run this query:
select id from page where (sortorder = (select sortorder from page where id = 1) - 1)
(I'm trying to find the previous page, ie the one with the lower sortorder, if it exists. If none exists, I want an empty result set.)
The error I receive from mysql:
SQL Error (1690): BIGINT UNSIGNED value is out of range in '((select '0' from `page` where 1) - 1)'
And more specifically when I run:
select sortorder - 1 from page where id = 1
I get:
SQL Error (1690): BIGINT UNSIGNED value is out of range in '('0' - 1)'
What can I do to prevent this?
I usually use JOINs for this goal because they can be optimized better than the sub-queries. This query should produce the same result as yours but probably faster:
SELECT pp.*
FROM page cp # 'cp' from 'current page'
LEFT JOIN page pp # 'pp' from 'previous page'
ON pp.sortorder = cp.sortorder - 1
WHERE cp.id = 1
Unfortunately it fails running with the same error message about -1 not being UNSIGNED.
It can be fixed by writing the JOIN condition as:
ON pp.sortorder + 1 = cp.sortorder
I moved the -1 to the other side of the equal sign and it turned to +1.
You can also fix your original query by using the same trick: moving -1 to the other side of the equal sign; this way it becomes +1 and there is no error any more:
select id
from page
where (sortorder + 1 = (select sortorder from page where id = 1)
The problem with both queries now is that, because there is no index on column sortorder, MySQL is forced to check all the rows one by one until it finds one matching the WHERE (or ON) condition and this takes a lot of time and uses a lot of resources.
Fortunately, this can be fixed easily by adding an index on column sortorder:
ALTER TABLE page ADD INDEX(sortorder);
Now both queries can be used. The one using JOIN (and the ON condition with +1) is slightly faster.
The original query doesn't return any rows when the condition is not met. The JOIN query returns a row full of NULLs. It can be modified to return no rows by replacing LEFT JOIN with INNER JOIN.
You can circumvent the error altogether (and use any version of these queries) by removing the UNSIGNED attribute from column sortorder:
ALTER TABLE page
CHANGE COLUMN `sortorder` `sortorder` SMALLINT(5) UNSIGNED NOT NULL;
Try to set your SQL Mode in 'NO_UNSIGNED_SUBTRACTION'
SET sql_mode = 'NO_UNSIGNED_SUBTRACTION'
I came across this strange behavior when I was hunting for a bug in a system. Consider following.
We have a mysql table which have varchar(100) column. See the following sql script.
create table user(`id` bigint(20) NOT NULL AUTO_INCREMENT,`user_id` varchar(100) NOT NULL,`username` varchar(255) DEFAULT NULL,PRIMARY KEY (`id`),UNIQUE KEY `user_id` (`user_id`)) ENGINE=InnoDB AUTO_INCREMENT=129 DEFAULT CHARSET=latin1;
insert into user(user_id, username) values('20120723145614834', 'user1');
insert into user(user_id, username) values('20120723151128642', 'user1');
When I execute following query I received 0 results.
select * from user where user_id=20120723145614834;
But When I execute following I get the result(note the single quote).
select * from user where user_id='20120723145614834';
This is expected since user_id field is varchar. Strange thing is that both following queries yield result.
select * from user where user_id=20120723151128642;
select * from user where user_id='20120723151128642';
Can anybody explain me the reason for this strange behavior. My MySql version is 5.1.63-0ubuntu0.11.10.1
Check mysql document 12.2. Type Conversion in Expression Evaluation
Comparisons that use floating-point numbers (or values that are
converted to floating-point numbers) are approximate because such
numbers are inexact. This might lead to results that appear
inconsistent:
mysql> SELECT '18015376320243458' = 18015376320243458;
-> 1
mysql> SELECT '18015376320243459' = 18015376320243459;
-> 0
So we better use always right data type for SQL.
Is there an easy and simple way to create a result table that has specified columns but zero rows? In set theory this is called an empty set, but since relational databases use multidimensional sets the term doesn't fit perfectly. I have tried these two queries, but both deliver exactly one row and not zero rows:
SELECT '' AS ID;
SELECT null AS ID;
But what I want is the same result as this query:
SELECT ID FROM sometable WHERE false;
I'm searching for a more elegant way because I don't want to have a table involved, so the query is independent from any database scheme. Also a generic query might be a bit faster (not that it would matter for such a query).
SELECT "ID" LIMIT 0;
Without any real tables.
Do note that most (My)SQL clients simply will display "Empty set". However, it actually does what you want:
create table test.test_table
select "ID" limit 0;
show create table test.test_table\G
Table: test_table
Create Table: CREATE TABLE `test_table` (
`ID` varchar(2) character set latin1 NOT NULL default ''
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin
SELECT * FROM (SELECT NULL AS ID) AS x WHERE 1 = 0
You can use the DUAL pseudo-table.
SELECT whatever FROM DUAL WHERE 1 = 0
Check the documentation (look for the DUAL section).
I've got this table
CREATE TABLE `subevents` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`title` varchar(150) DEFAULT NULL,
`content` text,
`class` tinyint(4) NOT NULL DEFAULT '1',
PRIMARY KEY (`id`)
) ENGINE=MyISAM
Each row can have a different value in the 'class' field.
I'd like to select any number of rows, ordered randomly, as long as the sum of the values in the 'class' field is equal to 100.
How could I accomplish it directly in the MySQL query without doing it later in PHP?
Thanks everybody!
By "ordered randomly" I assume you mean that the order of the rows doesn't matter but no row can be used more than once. So you are looking for a combination of rows in which the sum of class equals 100. Use the brute force method. Randomly generate possible solutions until you find one that works.
delimiter //
CREATE PROCEDURE subsetsum(total)
BEGIN
DECLARE sum INTEGER;
REPEAT
CREATE OR REPLACE VIEW `solution`
AS SELECT * FROM `subevents`
WHERE 0.5 <= RAND();
SELECT SUM(`class`) INTO sum FROM `solution`;
UNTIL sum = total END REPEAT;
END
//
delimiter ;
CALL subsetsum(100); /* For example */
SELECT * FROM `solution`;
I have tested this with tables having a TINYINT column of random values and it is actually reasonably fast. The only problem is that there is no guarantee that subsetsum() will ever return.
I don't think this is possible with only SQL...the only thing which comes to my mind is to redo a the sql query as long the sum isn't 100
But I have no clue how to select a random number of rows at once.