DROP TABLE (
SELECT table_name
FROM information_schema.`TABLES`
WHERE table_schema = 'myDatabase' AND table_name LIKE BINARY 'del%');
I know this doesn't work! What is the equivalent for something like this in SQL? I can whip out a simple Python script to do this but was just wondering if we can do something with SQL directly. I am using MySQL. Thank you!
You can use prepared statements -
SET #tables = NULL;
SELECT GROUP_CONCAT('`', table_schema, '`.`', table_name,'`') INTO #tables FROM information_schema.tables
WHERE table_schema = 'myDatabase' AND table_name LIKE BINARY 'del%';
SET #tables = CONCAT('DROP TABLE ', #tables);
PREPARE stmt1 FROM #tables;
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
It will generate and execute a statement like this -
DROP TABLE myDatabase.del1, myDatabase.del2, myDatabase.del3;
A minor improvement to #Devart's answer:
SET #tables = NULL;
SELECT GROUP_CONCAT(table_schema, '.`', table_name, '`') INTO #tables FROM
(select * from
information_schema.tables
WHERE table_schema = 'myDatabase' AND table_name LIKE 'del%'
LIMIT 10) TT;
SET #tables = CONCAT('DROP TABLE ', #tables);
select #tables;
PREPARE stmt1 FROM #tables;
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
This script should be executed repeatedly until the console's output is NULL
The changes are:
backtick (`) wrapping the table name (if it contains non standard characters)
added a LIMIT to avoid the truncation issue I commented about
added a "print" (select #tables;) to have some kind of control when to stop executing the script
If you just need to quickly drop a bunch of tables (not in pure SQL, so not directly answering this question) a one line shell command can do it:
echo "show tables like 'fsm%'" | mysql | tail +2 | while read t; do echo "drop table \`$t\`;"; done | mysql
I found it useful to add an IFNULL to Devart's solutions to avoid generating an error if there are no tables matching the query.
SET #tables = IFNULL(CONCAT('DROP TABLE ', #tables),'SELECT NULL;');
In addition to #Devart's answer:
If you have many tables, you may need to set group_concat_max_len:
SET group_concat_max_len = 4096;
You can do this quickly and easily if you have phpMyAdmin available and the requirement is to drop tables with a specific prefix. Go to the database you want, and show the list of all the tables. Since tables are shown in alphabetic order, the tables with the prefix for deletion will all appear together. Go to the first one, and click the tick box on the left hand side. Then scroll down to the last table with the prefix, hold down shift, and click the tick box. That results in all the tables with the prefix being ticked. Go to the bottom of the list, and select the action drop for all selected tables. Go through with the deletion, checking that the SQL generated looks right!
Related
Please explain me the below example
SELECT GROUP_CONCAT(COLUMN_NAME)
FROM information_schema.`COLUMNS` C
WHERE table_name = 'table_name'
AND COLUMN_NAME =('columns_name') INTO #COLUMNS;
SET #table = 'table_name';
SET #s = CONCAT('SELECT ',#columns,' FROM ', #table);
PREPARE stmt FROM #s;
This pattern is all about creating dynamic (prepared in MySQL parlance) queries based on the names of columns in a particular table. INFORMATION_SCHEMA is a built-in database with read-only tables describing all the tables in all databases on the MySQL server.
The first query in your sequence retrieves a text string in the local variable #COLUMNS with a value like
id,name,value,description
for a table named table_name with those four columns.
The third one retrieves a string in the local variable #s with a value containing a query like
SELECT id,name,value,description FROM table_name
The fourth one, PREPARE, gets ready to do EXECUTE stmt, which runs the query. You can read about PREPARE and EXECUTE here.
The whole sequence of queries in your question does almost exactly the same thing as SELECT * FROM table_name.
There's a defect in your first query. You should add AND TABLE_SCHEMA = DATABASE() to its WHERE clause. Otherwise, you may pick up columns from tables named table_name in multiple databases.
I try to use the following code to empty a database (delete all tables) :
SELECT concat('DROP TABLE IF EXISTS ','`',table_schema,'`','.','`',table_name,'`',';') FROM information_schema.tables WHERE table_schema = 'DB';
I am getting an output of the commands, but nothing happens to the database. If I take an individual command from the output, and run it in the console, it works.
What am I doing wrong?
As you know, SELECT only returns the result of a query. It doesn't know you actually intend to execute the result of that query. (In most cases, that would make no sense.) You can use prepared statements to do what you want (untested):
SET #s:='';
SELECT #s:=concat(#s, 'DROP TABLE IF EXISTS ','`',table_schema,'`','.','`',table_name,'`',';') FROM information_schema.tables WHERE table_schema = 'DB';
PREPARE stm FROM #s;
EXECUTE stm;
DEALLOCATE PREPARE stm;
Why you're using SELECT statement, Simply try this query:
DROP TABLE IF EXISTS table_name;
I've found another thread on this question, but I wasn't able to use its solutions, so I thought I'd ask with more clarity and detail.
I have a large MySQL database representing a vBulletin forum. For several years, this forum has had an error generated on each view, each time creating a new table named aagregate_temp_1251634200, aagregate_temp_1251734400, etc etc. There are about 20,000 of these tables in the database, and I wish to delete them all.
I want to issue a command that says the equivalent of DROP TABLE WHERE TABLE_NAME LIKE 'aggregate_temp%';.
Unfortunately this command doesn't work, and the Google results for this problem are full of elaborate stored procedures beyond my understanding and all seemingly tailored to the more complex problems of different posters.
Is it possible to write a simple statement that drops multiple tables based on a name like match?
There's no single statement to do that.
The simplest approach is to generate a set of statements, and execute them individually.
We can write a simple query that will generate the statements for us:
SELECT CONCAT('DROP TABLE `',t.table_schema,'`.`',t.table_name,'`;') AS stmt
FROM information_schema.tables t
WHERE t.table_schema = 'mydatabase'
AND t.table_name LIKE 'aggregate\_temp%' ESCAPE '\\'
ORDER BY t.table_name
The SELECT statement returns a rowset, but each row conveniently contains the exact SQL statement we need to execute to drop a table. (Note that information_schema is a builtin database that contains metadata. We'd need to replace mydatabase with the name of the database we want to drop tables from.
We can save the resultset from this query as a plain text file, remove any heading line, and voila, we've got a script we can execute in our SQL client.
There's no need for an elaborate stored procedure.
A little googling found this:
SELECT 'DROP TABLE "' + TABLE_NAME + '"'
FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_NAME LIKE 'prefix%'
This should generate a script.
Source: Drop all tables whose names begin with a certain string
From memory you have to use prepared statements, for example: plenty of samples on stack exchange
I would recommend this example:
SQL: deleting tables with prefix
The SQL from above, this one includes the specific databasename - it builds it for you
SELECT CONCAT( 'DROP TABLE ', GROUP_CONCAT(table_name) , ';' )
AS statement FROM information_schema.tables
WHERE table_schema = 'database_name' AND table_name LIKE 'myprefix_%';
Here is a different way to do it:
MySQL bulk drop table where table like?
This will delete all tables with prefix "mg_"
No need to copy and paste rowsets and in phpadmin copying and pasting is problematic as it will cut off long table names and replace them with '...' ruining set of sql commands.
Also note that '_' is a special character so thats why 'mg_' should be encoded as 'mg\_'
(and FOREIGN_KEY_CHECKS needs to be disabled in order to avoid error messages)
SET FOREIGN_KEY_CHECKS = 0;
SET GROUP_CONCAT_MAX_LEN=32768;
SET #tables = NULL;
SELECT GROUP_CONCAT('`', table_name, '`') INTO #tables
FROM information_schema.tables
WHERE table_schema = (SELECT DATABASE()) and table_name like 'mg\_%';
SELECT IFNULL(#tables,'dummy') INTO #tables;
SET #tables = CONCAT('DROP TABLE IF EXISTS ', #tables);
PREPARE stmt FROM #tables;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET FOREIGN_KEY_CHECKS = 1;
After some searching here on stackoverflow and on the web, I couldn't find the answer to my question. I'm not a real SQL talent, but I'm trying to covert all the columns in my table to varchar (255). It has about 600 columns which are all varchar, but the size limit varies. I would like them all to be 255. Is there a way to not having to do this manually? I work with MySQL.
Thanks!
You need to generate the alter table statement by pulling the data from the database.
select 'alter table MyTableName modify column ' + column_name + ' varchar(255);'
from information_schema where table_name = 'MyTableName'
And then paste the results of this command into the query window and run it -- making sure it does what you want it to do. Do a backup first.
Or you could make one big alter statement (if MySql wouldn't choke on it) by replacing the semicolon with a comma.
This isn't what you really need to do. You have something more important to do: NORMALIZE YOUR DATABASE
Now, It's impossible that you have a normalized table with 600 columns. Split your entities in that table correctly, following at least the 3rd normal form rules. After that, you'll have a much better database which is easier to mantain.
To do this, you'll need to drop your current table, therefore, you don't need to change all the types to varchar(255) because you'll fix them during the creation of other tables.
This would be a good start to read: http://en.wikipedia.org/wiki/Database_normalization (thanks to #Tim Schmelter from question's comments)
First of all as mentioned by others you better off normalize you data.
In the meantime you can achieve your goal with dynamic SQL like this
DELIMITER $$
CREATE PROCEDURE change_to_varchar255(IN _tname VARCHAR(64))
BEGIN
SET #sql = NULL;
SELECT GROUP_CONCAT(
CONCAT_WS(' ', 'CHANGE', COLUMN_NAME, COLUMN_NAME, 'VARCHAR(255)'))
INTO #sql
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = _tname
AND DATA_TYPE = 'varchar'
AND CHARACTER_MAXIMUM_LENGTH < 255
AND TABLE_SCHEMA = SCHEMA();
SET #sql = CONCAT_WS(' ', 'ALTER TABLE', _tname, #sql);
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END$$
DELIMITER ;
Sample usage:
CALL change_to_varchar255('table1');
Here is SQLFiddle demo
If you are using PhpMyAdmin or other, you can also click on the button to modify the table.
When you are on the web page, press Ctrl+Shift+J under Windows or Cmd+Opt+J under Mac to open the console window in the Chrome Developer tools. Now enter the following command to replace all occurrences of the number 255 with 100 :
document.body.innerHTML = document.body.innerHTML.replace(/255/g, "100").
Finally, click on the button to execute the query.
I have 100 tables in my database, I want to keep one only.
I tried something like below query:
DROP ALL TABLES EXCEPT my_table
But that doesn't seem to exist. any idea?
You can build a DROP TABLE statement with multiple listed tables, and run the query using MySQL prepared statements -
SET #tables = NULL;
SELECT GROUP_CONCAT(table_schema, '.', table_name) INTO #tables FROM information_schema.tables
WHERE table_schema = 'Database1' AND table_name <> 'my_table';
SET #tables = CONCAT('DROP TABLE ', #tables);
PREPARE stmt1 FROM #tables;
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
You can't drop multiple tables in MySQL.
The easiest in your case would be to export the table you want to keep (using a tool like mysqldump), then drop and recreate the database.
I wonder why no one suggested to go to the directory :
C:\Program Files (x86)\Parallels\Plesk\Databases\MySQL\data\user_db\...
and delete them easily there? this is what I did and it seems working
If you have lots of tables, watch out for truncation by the GROUP_CONCAT function. It has a setting for maximum length of the returned string: group_concat_max_len
See: GROUP_CONCAT reference manual entry
This answer guided me to the solution:
MYSQL: How to define or get LONG string variables
You can build all drop statements output them info file and use file after that
SELECT CONCAT( 'DROP TABLE ', table_name , ';' ) AS statement FROM information_schema.tables WHERE table_name not like 'table_to_keep' and table_schema like 'your_schema' into outfile 'path_to_file;
source path_to_file;
You can use mysqldump , try it !
mysqldump -u[USERNAME] -p[PASSWORD]--add-drop-table --no-data [DATABASE] |
grep ^DROP |
grep -v 'my_table' |
mysql -u[USERNAME] -p[PASSWORD] [DATABASE]