Mysql – Should I use left join to do the job in this scenario

join;MySQLperformancequery-performance

What I want to do is to find out the items which appeared more than once in this table:

mysql> desc tables;

+-----------------+---------------------+------+-----+---------+-------+
| Field           | Type                | Null | Key | Default | Extra |
+-----------------+---------------------+------+-----+---------+-------+
| TABLE_CATALOG   | varchar(512)        | YES  |     | NULL    |       | 
| TABLE_SCHEMA    | varchar(64)         | NO   |     |         |       | 
| TABLE_NAME      | varchar(64)         | NO   |     |         |       | 
| TABLE_TYPE      | varchar(64)         | NO   |     |         |       | 
| ENGINE          | varchar(64)         | YES  |     | NULL    |       | 
| VERSION         | bigint(21) unsigned | YES  |     | NULL    |       | 
| ROW_FORMAT      | varchar(10)         | YES  |     | NULL    |       | 
| TABLE_ROWS      | bigint(21) unsigned | YES  |     | NULL    |       | 
| AVG_ROW_LENGTH  | bigint(21) unsigned | YES  |     | NULL    |       | 
| DATA_LENGTH     | bigint(21) unsigned | YES  |     | NULL    |       | 
| MAX_DATA_LENGTH | bigint(21) unsigned | YES  |     | NULL    |       | 
| INDEX_LENGTH    | bigint(21) unsigned | YES  |     | NULL    |       | 
| DATA_FREE       | bigint(21) unsigned | YES  |     | NULL    |       | 
| AUTO_INCREMENT  | bigint(21) unsigned | YES  |     | NULL    |       | 
| CREATE_TIME     | datetime            | YES  |     | NULL    |       | 
| UPDATE_TIME     | datetime            | YES  |     | NULL    |       | 
| CHECK_TIME      | datetime            | YES  |     | NULL    |       | 
| TABLE_COLLATION | varchar(32)         | YES  |     | NULL    |       | 
| CHECKSUM        | bigint(21) unsigned | YES  |     | NULL    |       | 
| CREATE_OPTIONS  | varchar(255)        | YES  |     | NULL    |       | 
| TABLE_COMMENT   | varchar(80)         | NO   |     |         |       | 
+-----------------+---------------------+------+-----+---------+-------+
21 rows in set (0.02 sec)

mysql> select count(table_name) from tables;

+-------------------+
| count(table_name) |
+-------------------+
|              2704 | 
+-------------------+
1 row in set (5.33 sec)

mysql> select count(distinct table_name) from tables; 

+----------------------------+
| count(distinct table_name) |
+----------------------------+
|                       1427 | 
+----------------------------+
1 row in set (3.16 sec)

first query:

select table_name as a from tables left join (select distinct table_name as b from tables) on a != b;

seems this query is quite inefficient, someone said left join could be better,
select table_name from tables where table_name not in (select distinct table_name from tables);

what is command means? first select a set (we name it set A)of 2704 items, then select the other set(we name it set B) which have 1427 items.
then run 2704 times to assure if the item in set A is in set B, if not, then that item will be added to the result set.

so how many comparation will be operated by mysql engine?is that 2704 x 1427?

or mysql engine may have some smart way to deal with this?

method 2 not works either:

mysql> select count(table_name) as num, table_name from tables group by table_name where num > 1;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'where num > 1' at line 1
mysql> select count(table_name) as num, table_name from tables where num>1 group by table_name ;             
ERROR 1054 (42S22): Unknown column 'num' in 'where clause'
mysql> select count(table_name) as num, table_name from tables group by table_name where num > 1;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'where num > 1' at line 1

select count(table_name) as num, table_name from tables group by table_name having num > 1;

this query works well, why the former queries failed?

why this left join failed?

mysql> select table_name from tables as a left join (select distinct table_name from tables as b)  on a.table_name!=b.table_name;
ERROR 1248 (42000): Every derived table must have its own alias

Best Answer

Your last query

mysql> select table_name from tables as a left join (select distinct table_name from tables as b)  on a.table_name!=b.table_name;
ERROR 1248 (42000): Every derived table must have its own alias

fails because the as b must be outside the parentheses like this

mysql> select table_name from tables as a left join (select distinct table_name from tables) as b  on a.table_name!=b.table_name;

Your method 2 failed

mysql> select count(table_name) as num, table_name from tables group by table_name where num > 1;

because the group by table_name must be last like this

mysql> select count(table_name) as num, table_name from tables where num > 1 group by table_name;

SUGGESTION

Perhaps you should start with a GROUP BY ... HAVING query like this

SELECT table_name,COUNT(1) table_count
FROM information_schema.tables
WHERE table_schema NOT IN
('information_schema','performance_schema','mysql')
GROUP BY table_name HAVING COUNT(1) > 1;

This will definitely give all tables whose name appears in multiple databases

Form that as a subquery and join it to gather all databases the table appears in

SELECT
    A.table_name,
    GROUP_CONCAT(B.table_schema) TheTableAppearsInTheseDatabases
FROM
(
    SELECT table_name,COUNT(1) table_count
    FROM information_schema.tables
    WHERE table_schema NOT IN
    ('information_schema','performance_schema','mysql')
    GROUP BY table_name HAVING COUNT(1) > 1
) A INNER JOIN information_schema.tables B
USING (table_name) GROUP BY A.table_name;

Please notice that I use INNER JOIN rather than LEFT JOIN because it will really form a Cartesian product (2704 X 2704) and then perform comparisons.

I know this works because I tried it out in MySQL 5.5.12 on my Windows7 machine

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mysql>     SELECT
    ->         A.table_name,
    ->         GROUP_CONCAT(B.table_schema) TheTableAppearsInTheseDatabases
    ->     FROM
    ->     (
    ->         SELECT table_name,COUNT(1) table_count
    ->         FROM information_schema.tables
    ->         WHERE table_schema NOT IN
    ->         ('information_schema','performance_schema','mysql')
    ->         GROUP BY table_name HAVING COUNT(1) > 1
    ->     ) A INNER JOIN information_schema.tables B
    ->     USING (table_name) GROUP BY A.table_name;
+--------------------------+-------------------------------------------------------------------------------------------------------------------+
| table_name               | TheTableAppearsInTheseDatabases                                                                                   |
+--------------------------+-------------------------------------------------------------------------------------------------------------------+
| a                        | junk,test,robottinosino                                                                                           |
| acl                      | weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive                                     |
| blocks                   | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| blog                     | weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive                                     |
| blog_category            | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| blog_entrycat            | weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive                                     |
| blog_meta                | weisci_jaws_staging,weisci_jaws_live,weisci_jaws_archive,weisci_jaws_staging2                                     |
| blog_trackback           | weisci_jaws_archive,weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_staging                                     |
| calendar_events          | weisci_jaws_staging,weisci_jaws_archive,weisci_jaws_live,weisci_jaws_staging2                                     |
| calendar_meta            | weisci_jaws_archive,weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_staging                                     |
| calendar_questions       | weisci_jaws_staging,weisci_jaws_archive,weisci_jaws_live,weisci_jaws_staging2                                     |
| calendar_tickets         | weisci_jaws_archive,weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_staging                                     |
| calendar_transactions    | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_staging,weisci_jaws_archive                                     |
| captcha_complex          | weisci_jaws_staging,weisci_jaws_archive,weisci_jaws_live,weisci_jaws_staging2                                     |
| change_log               | test,junk                                                                                                         |
| chat_staff               | test,junk                                                                                                         |
| comments                 | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_staging,weisci_jaws_archive                                     |
| donations_charities      | weisci_jaws_staging,weisci_jaws_archive,weisci_jaws_live,weisci_jaws_staging2                                     |
| donations_charities_meta | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| donations_donations      | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| foo_reference1           | timpost1,timpost2                                                                                                 |
| foo_reference2           | timpost1,timpost2                                                                                                 |
| foo_reference3           | timpost1,timpost2                                                                                                 |
| groups                   | weisci_jaws_staging,weisci_jaws_archive,weisci_jaws_live,weisci_jaws_staging2                                     |
| ipvisitor                | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| job_post                 | giannosfor,test                                                                                                   |
| layout                   | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| listeners                | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| mediamanager_files       | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_staging                                                         |
| mediamanager_group       | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| mediamanager_photos      | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| mediamanager_video       | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| menus                    | weisci_jaws_archive,weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2                                     |
| menus_groups             | weisci_jaws_archive,weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2                                     |
| mytable                  | ryanzec,user1267617,cabita,dotancohen,johnlocke,neeraj,test,user391986,cool_cs,javier,mathieu                     |
| mytext                   | jakobud,newstuff                                                                                                  |
| occupation_field         | giannosfor,test                                                                                                   |
| policy_agentblock        | weisci_jaws_archive,weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2                                     |
| policy_ipblock           | weisci_jaws_archive,weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2                                     |
| prova                    | veto,vito                                                                                                         |
| registry                 | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| registry_bk              | weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2                                                         |
| session                  | weisci_jaws_archive,weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2                                     |
| static_pages             | weisci_jaws_archive,weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2                                     |
| static_pages_translation | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| t                        | preeti,rollup_test                                                                                                |
| t1                       | abidibo,test                                                                                                      |
| t2                       | test,abidibo                                                                                                      |
| t3                       | test,abidibo                                                                                                      |
| table1                   | table_test,supercoolville,test                                                                                    |
| table2                   | supercoolville,test,table_test                                                                                    |
| tags                     | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| tags_content             | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| tbl_banner_position      | weisci_jaws_staging2,weisci_jaws_live                                                                             |
| tbl_banner_upload        | weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2                                                         |
| tbl_global_banner        | weisci_jaws_staging2,weisci_jaws_live                                                                             |
| tms_authors              | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| tms_repositories         | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| tms_themes               | weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging                                     |
| updates                  | test,junk                                                                                                         |
| url_aliases              | weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2,weisci_jaws_archive                                     |
| url_maps                 | weisci_jaws_archive,weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2                                     |
| users                    | weisci_jaws_archive,weisci_jaws_staging,giannosfor,veto,weisci_jaws_live,weisci_jaws_staging2,friends,sample,vito |
| users_groups             | weisci_jaws_archive,weisci_jaws_staging,weisci_jaws_live,weisci_jaws_staging2                                     |
| users_meta               | weisci_jaws_staging2,weisci_jaws_archive,weisci_jaws_staging,weisci_jaws_live                                     |
+--------------------------+-------------------------------------------------------------------------------------------------------------------+
65 rows in set (0.91 sec)

mysql>

Give it a Try !!!

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