I'm creating a function in mySQL that will update table metrics each time the any table has something inserted or deleted from it. So I've got my Table_metrics table, with elements table_name and row_count:
CREATE TABLE Table_metrics (
table_name VARCHAR(64),
row_count INTEGER DEFAULT 0 );
So any time something gets added to any of my tables (other than this one) in the database the matching row gets updated with the number of rows in that table.
To do this I've tried making a stored procedure:
CREATE PROCEDURE table_update_metric(IN tablename VARCHAR(64))
UPDATE Table_metrics
SET row_count=(SELECT COUNT(*) FROM tablename)
WHERE table_name = tablename;
This produces an error when I call the procedure call table_update_metric('Owners'); (Owners being a table in my database)
table DB.tablename doesn't exist
I've done a bit of digging into stored procedures, trying to figure out how they would work. I assume the issue is coming from the line `SELECT COUNT(*) FROM tablename), so I tried having a stored statement in the procedure:
CREATE PROCEDURE table_update_metric(IN TABLENAME VARCHAR(64))
SET #s = CONCAT('SELECT COUNT(*) FROM ', tablename)
UPDATE Table_metrics
SET row_count=EXECUTE #s
WHERE table_name = tablename;
I'm not really sure how to properly do a stored statement as I'm still relatively new to mySQL, but I believe that it's the way to go.
Can anyone offer any insight into this problem?
What you are trying to do is already provided in Mysql. Information Schema Tables stores this information for you :)
SELECT TABLE_NAME, TABLE_ROWS)
FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_SCHEMA = '{your_db}';
From what Somonare said, I created a table in my database that uses that information:
CREATE TABLE Table_metrics
SELECT TABLE_NAME, TABLE_ROWS FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_SCHEMA = 'Sensors';
And this table will be updated with a trigger each time the tables in Sensors are updated.
Related
I have here a mysql script which alters my 'Job' table and changes the DESCRIPTION column data type to TEXT. However, I have this script together with all the other scripts which are sometimes run multiple times.
My question is, what do I need to add to my script so that it would check if the data type of DESCRIPTION column is already TEXT or not? This script takes too long to execute due to huge data and I don't want it to be executed again if the DESCRIPTION column has already been changed to TEXT.
ALTER TABLE Job
MODIFY DESCRIPTION TEXT;
SELECT
COLUMN_NAME, DATA_TYPE
FROM
INFORMATION_SCHEMA.COLUMNS
WHERE
TABLE_SCHEMA = 'YOUR_DB_NAME'
AND
TABLE_NAME = 'YOUR_TABLE_NAME'
AND
COLUMN_NAME = 'YOUR_COLUMN_NAME';
This will give you datatype of asked column.
Using If condition you can run your alter table command;
Here's what I did. It's a long way but it worked. It needs to be in a stored procedure. Thanks for your help #shahmanthan9. If you guys know a better way please post it here. Thanks!
DROP PROCEDURE IF EXISTS sp_JobUpdateDescriptionColumnType;
CREATE PROCEDURE sp_JobUpdateDescriptionColumnType()
DETERMINISTIC
SQL SECURITY DEFINER
COMMENT ''
BEGIN
IF NOT EXISTS( SELECT NULL
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_SCHEMA = DATABASE()
AND TABLE_NAME = 'Job'
AND COLUMN_NAME = 'Description'
AND DATA_TYPE = 'text' )
THEN
ALTER TABLE Job
MODIFY Description TEXT;
END IF;
END;
CALL sp_JobUpdateDescriptionColumnType;
DROP PROCEDURE IF EXISTS sp_JobUpdateDescriptionColumnType;
Im having a database which is over 60k tables and i want to delete all the tables that have 1 or 2 rows.
You can use the below code in SQL Server 2012. It will delete all the tables which is having row_count less than 3.
USE [YourDB]
GO
DECLARE #Max int, #Count int,#Table_Name Varchar(20)
SET #Max =0
IF OBJECT_ID('tempdb..#temp') IS NOT NULL
DROP TABLE #temp
CREATE TABLE #temp
(
table_name sysname ,
row_count INT,
reserved_size VARCHAR(50),
data_size VARCHAR(50),
index_size VARCHAR(50),
unused_size VARCHAR(50)
)
IF OBJECT_ID('tempdb..#temp1') IS NOT NULL
DROP TABLE #temp1
CREATE TABLE #temp1
(
ID int IDENTITY(1,1),
table_name sysname ,
row_count INT
)
SET NOCOUNT ON
INSERT #temp
EXEC sp_msforeachtable 'sp_spaceused ''?'''
INSERT INTO #temp1
SELECT a.table_name,
a.row_count
FROM #temp a
INNER JOIN information_schema.columns b
ON a.table_name collate database_default
= b.table_name collate database_default
GROUP BY a.table_name, a.row_count
HAVING a.row_count <3
SET #Count =(SELECT COUNT(*) FROM #temp1)
WHILE #Count > #Max
BEGIN
SET #Max = #Max +1
SET #Table_Name = (SELECT table_name FROM #temp1 WHERE ID = #Max)
EXEC('DROP TABLE ' +#Table_Name)
END
Use a MySQL-GUI, order by number of rows and drop all tables with 1-2 rows. it is as easy as deleting files in a windows folder. this
would take ~10 seconds + sort and drop time
or
Select table names of tables with 1-2 rows from information_schemas, load into an excel file and build your drop statements. takes around 2-5 minutes + drop time
or
Build a stored procedure that uses a Cursor to parse all the relevant table names into variables of your drop statement (like Hansa mentioned). this makes sense if you want to repeat your process from time to time. takes around 1-12 hours for beginners (depending on knowledge level) + drop time
Since logging in on SO probably took more time than solution 1 would,
i would recommend that solution.
In case you want to spend more time , the following query will show all table names for tables with 1-2 rows:
SELECT table_schema, table_name From information_schema.tables
WHERE table_schema='your_schema' # use your table_schema here
AND table_rows BETWEEN 1 and 2 ;
I would do it within some application side logic, using a programming language of your choice (which offers a mysql API).
Get all table names, described here, grouping them by table names
e.g. this could look something like:
SELECT COUNT(table_rows), table_name
FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_SCHEMA = '{your_db}'
GROUP BY table_name;
(this is not tested, but to give you a basic idea...)
Execute a delete statement for the tables with rowcount 2 or lower. Should be an easy task as soon as you got the results in your programming language (btw SQL statement to delete them is "DROP TABLE")
Note: I think it should be also possible using a SQL cursor, but as I said I would prefer the above solution.
You can try this.
First take all the table names from the database.
After getting all the table names put them in a array and execute a foreach loop that checks the num of rows for each table, something like this
$tables = array();
foreach ($tables as $table_name){
$query = "select * from '$table_name'";
$result = mysqli_query($dbCon, $query);
$num_rows = mysql_num_rows($result);
if($num_rows < 3){
$delete = "drop table $table_name";
$res_del = mysqli_query($dbCon, $delete);
echo $table_name." Deleted";
}
}
You probably cant do this with 1 SQL Statement unless you are using stored procedures (see below).
You will need to select all tables in MySQL with a programming language (e.g. Java with JDBC) using the following statement:
select TABLE_NAME from INFORMATION_SCHEMA.TABLES where TABLE_ROWS <= 2 AND TABLE_ROWS >= 1
Then you can use the results in the programming languge to issue drop Statements for each result record in a loop:
drop table <tablename>
For doing this with a stored procedure (i.e. without a programming language)
see the third comment on this page: MySQL Reference Drop Table.
Of course you will need to adapt this to your needs because this example does not select tables by their row numbers but by their names.
This is the query i am using:
IF EXISTS(SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE COLUMN_NAME =( N'CustomerVariable1Value'))
begin Alter table temp.DIM_BE_ACCOUNT drop column CustomerVariable1Value
It works fine the first time but when I run it again, it shows error.
How to make it error free and executes it many number of times?
Error message:
ALTER TABLE DROP COLUMN failed because column 'CustomerVariable1Value' does not exist in table 'DIM_BE_ACCOUNT'.
You are only looking for a column name out of all column names in the entire MySQL instance. You need to also filter by schema (=database) and table names:
IF EXISTS(SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE COLUMN_NAME = N'CustomerVariable1Value' AND TABLE_NAME = 'MyTableName' AND TABLE_SCHEMA = 'MyDatabase')
Here is a solution that does not involve querying INFORMATION_SCHEMA, it simply ignores the error if the column does not exist.
DROP PROCEDURE IF EXISTS `?`;
DELIMITER //
CREATE PROCEDURE `?`
(
)
BEGIN
DECLARE CONTINUE HANDLER FOR SQLEXCEPTION BEGIN END;
ALTER TABLE `table_name` DROP COLUMN `column_name`;
END //
DELIMITER ;
CALL `?`();
DROP PROCEDURE `?`;
P.S. Feel free to give it other name rather than ?
Working only with MySQL (I have essentially no PHP knowledge), I need to have a table that's essentially a subset from a much larger table. The source table changes from time to time, losing some entries, gaining other new ones, and values changing for existing ones. I can describe what I want to happen, but can't seem to figure out a syntax of commands to make it work. I also know I can have two separate queries and just run whichever one I need, and I have that worked out, but I'd like to combine them if possible. Here's what I want:
IF the subset_table DOES NOT EXIST, create it as [select query], ELSE truncate the subset_table and insert [select query]
Like I said, I know there are other ways to do this - I could drop if exists/create, or I could just have two different sql files to run. I just want to know if I can do this as specified above.
Thoughts?
You can do this:
create table if not exists <tablename> . . .;
truncate table <tablename>;
insert into <tablename>(cols)
select blah blahblah . . .;
You don't need any if statements at all.
This can also be done through an SP (stored procedure)... makes it more readable and safe
DELIMITER $$
DROP PROCEDURE IF EXISTS `create_table_sp`$$
CREATE PROCEDURE `create_table_sp`()
BEGIN
IF NOT EXISTS (SELECT 1 FROM information_schema.TABLES WHERE table_name = '<table_name>'
AND table_schema = DATABASE() AND table_type = 'BASE TABLE') THEN
CREATE TABLE <subset_table_name>
AS SELECT * FROM <main_table_name>;
ELSE
TRUNCATE TABLE <subset_table_name>;
INSERT INTO <subset_table_name>
AS SELECT * FROM <main_table_name>;
END IF;
END$$
DELIMITER ;
CALL `create_table_sp`;
DROP PROCEDURE IF EXISTS `create_table_sp`;
There is also another way,
You could pass the table names as arguments to the SP, in this case sub_table_name and main_table_name
Make the above DML statements to a string using CONCAT()
Create a prepared statement out of it and execute
Hope this helped....
How can ALTER be used to drop a column in a MySQL table if that column exists?
I know I can use ALTER TABLE my_table DROP COLUMN my_column, but that will throw an error if my_column does not exist. Is there alternative syntax for dropping the column conditionally?
I'm using MySQL version 4.0.18.
For MySQL, there is none: MySQL Feature Request.
Allowing this is arguably a really bad idea, anyway: IF EXISTS indicates that you're running destructive operations on a database with (to you) unknown structure. There may be situations where this is acceptable for quick-and-dirty local work, but if you're tempted to run such a statement against production data (in a migration etc.), you're playing with fire.
But if you insist, it's not difficult to simply check for existence first in the client, or to catch the error.
MariaDB also supports the following starting with 10.0.2:
DROP [COLUMN] [IF EXISTS] col_name
i. e.
ALTER TABLE my_table DROP IF EXISTS my_column;
But it's arguably a bad idea to rely on a non-standard feature supported by only one of several forks of MySQL.
There is no language level support for this in MySQL. Here is a work-around involving MySQL information_schema meta-data in 5.0+, but it won't address your issue in 4.0.18.
drop procedure if exists schema_change;
delimiter ';;'
create procedure schema_change() begin
/* delete columns if they exist */
if exists (select * from information_schema.columns where table_schema = schema() and table_name = 'table1' and column_name = 'column1') then
alter table table1 drop column `column1`;
end if;
if exists (select * from information_schema.columns where table_schema = schema() and table_name = 'table1' and column_name = 'column2') then
alter table table1 drop column `column2`;
end if;
/* add columns */
alter table table1 add column `column1` varchar(255) NULL;
alter table table1 add column `column2` varchar(255) NULL;
end;;
delimiter ';'
call schema_change();
drop procedure if exists schema_change;
I wrote some more detailed information in a blog post.
I know this is an old thread, but there is a simple way to handle this requirement without using stored procedures. This may help someone.
set #exist_Check := (
select count(*) from information_schema.columns
where TABLE_NAME='YOUR_TABLE'
and COLUMN_NAME='YOUR_COLUMN'
and TABLE_SCHEMA=database()
) ;
set #sqlstmt := if(#exist_Check>0,'alter table YOUR_TABLE drop column YOUR_COLUMN', 'select ''''') ;
prepare stmt from #sqlstmt ;
execute stmt ;
Hope this helps someone, as it did me (after a lot of trial and error).
I just built a reusable procedure that can help making DROP COLUMN idempotent:
-- column_exists:
DROP FUNCTION IF EXISTS column_exists;
DELIMITER $$
CREATE FUNCTION column_exists(
tname VARCHAR(64),
cname VARCHAR(64)
)
RETURNS BOOLEAN
READS SQL DATA
BEGIN
RETURN 0 < (SELECT COUNT(*)
FROM `INFORMATION_SCHEMA`.`COLUMNS`
WHERE `TABLE_SCHEMA` = SCHEMA()
AND `TABLE_NAME` = tname
AND `COLUMN_NAME` = cname);
END $$
DELIMITER ;
-- drop_column_if_exists:
DROP PROCEDURE IF EXISTS drop_column_if_exists;
DELIMITER $$
CREATE PROCEDURE drop_column_if_exists(
tname VARCHAR(64),
cname VARCHAR(64)
)
BEGIN
IF column_exists(tname, cname)
THEN
SET #drop_column_if_exists = CONCAT('ALTER TABLE `', tname, '` DROP COLUMN `', cname, '`');
PREPARE drop_query FROM #drop_column_if_exists;
EXECUTE drop_query;
END IF;
END $$
DELIMITER ;
Usage:
CALL drop_column_if_exists('my_table', 'my_column');
Example:
SELECT column_exists('my_table', 'my_column'); -- 1
CALL drop_column_if_exists('my_table', 'my_column'); -- success
SELECT column_exists('my_table', 'my_column'); -- 0
CALL drop_column_if_exists('my_table', 'my_column'); -- success
SELECT column_exists('my_table', 'my_column'); -- 0
Chase Seibert's answer works, but I'd add that if you have several schemata you want to alter the SELECT thus:
select * from information_schema.columns where table_schema in (select schema()) and table_name=...
You can use this script, use your column, schema and table name
IF EXISTS (SELECT *
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = 'TableName' AND COLUMN_NAME = 'ColumnName'
AND TABLE_SCHEMA = SchemaName)
BEGIN
ALTER TABLE TableName DROP COLUMN ColumnName;
END;
Perhaps the simplest way to solve this (that will work) is:
CREATE new_table AS SELECT id, col1, col2, ... (only the columns you actually want in the final table)
FROM my_table;
RENAME my_table TO old_table, new_table TO my_table;
DROP old_table;
Or keep old_table for a rollback if needed.
This will work but foreign keys will not be moved. You would have to re-add them to my_table later; also foreign keys in other tables that reference my_table will have to be fixed (pointed to the new my_table).
Good Luck...
I realise this thread is quite old now, but I was having the same problem.
This was my very basic solution using the MySQL Workbench, but it worked fine...
get a new sql editor and execute SHOW TABLES to get a list of your tables
select all of the rows, and choose copy to clipboard (unquoted) from the context menu
paste the list of names into another editor tab
write your query, ie ALTER TABLE x DROP a;
do some copying and pasting, so you end up with separate query for each table
Toggle whether the workbench should stop when an error occurs
Hit execute and look through the output log
any tables which had the table now haven't
any tables which didn't will have shown an error in the logs
then you can find/replace 'drop a' change it to 'ADD COLUMN b INT NULL' etc and run the whole thing again....
a bit clunky, but at last you get the end result and you can control/monitor the whole process and remember to save you sql scripts in case you need them again.