I'm trying to write a MySQL stored proceedure that loops through all existing tables in my database and creates a copy/clone of each table. I'm using a cursor to loop through the table names then create a new table like this:
DELIMITER //
CREATE PROCEDURE CopyTables()
BEGIN
DECLARE finished INT DEFAULT 0;
DECLARE tableName VARCHAR(100);
DECLARE copyTableName VARCHAR(100);
DECLARE curTables
CURSOR FOR
SELECT table_name FROM INFORMATION_SCHEMA.TABLES;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET finished = 1;
OPEN curTables;
create_loop: LOOP
FETCH curTables INTO tableName;
IF finished THEN LEAVE create_loop; END IF;
SELECT concat('Processing table ', tableName);
SET copyTableName = CONCAT('copy_',tableName);
SELECT concat('Creating table ', copyTableName);
CREATE TABLE copyTableName LIKE tableName;
END LOOP;
CLOSE curTables;
END //
DELIMITER;
But I get an error when calling the stored procedure:
> call CopyTables()
[2020-12-08 18:16:03] 1 row retrieved starting from 1 in 77 ms (execution: 15 ms, fetching: 62 ms)
[2020-12-08 18:16:03] [S1000] Attempt to close streaming result set com.mysql.cj.protocol.a.result.ResultsetRowsStreaming#7a714591 that was not registered. Only one streaming result set may be open and in use per-connection. Ensure that you have called .close() on any active result sets before attempting more queries.
Is the result set exception effectively complaining because I'm creating new tables which is effectively messing with the cursor/select? I've got additional table changes on both the original and copied table to perform, like adding new columns, creating triggers, modifying constraints.
The list of table names is not static, and this should be able to run on whatever database I need it.
Can you suggest another way to achieve this without the cursor perhaps?
The problem is that the procedure is returning multiple result sets, but your Java client is not handling that correctly.
Refer to How do you get multiple resultset from a single CallableStatement?
Another problem with your procedure is that you aren't creating tables the way you think you are.
This statement:
CREATE TABLE copyTableName LIKE tableName;
will only create a table named literally copyTableName that is like another table that is literally tableName. It will NOT use the values of variables by those names.
To do what you want, you need to use a prepared statement:
SET #sql = CONCAT('CREATE TABLE `', copyTableName, '` LIKE `', tableName, '`');
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
This way the value of your variables is concatenated into an SQL statement.
Note that PREPARE only accepts a user-defined session variable, the type with the # sigil. It doesn't work with local variables you create in your procedure with DECLARE. Read https://dev.mysql.com/doc/refman/8.0/en/prepare.html and https://dev.mysql.com/doc/refman/8.0/en/user-variables.html
I am trying to add new event to rename a table in my DB use by phpmyadmine, and it will give me the errors :
1064 - You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near SET #tablename = 'Today Raw Data';SET #newname = ' raw Data'; SELECT #query' at line3
my code is :
select #yesterday := date_sub(curdate(),interval 1 day);
SET #tablename = 'Today Raw Data';
SET #newname = ' raw Data';
SELECT #query := CONCAT('RENAME TABLE `', #tablename, '` TO `',#yesterday,#newname, '`');
PREPARE STMT FROM #query;
EXECUTE STMT;
CREATE TABLE alter (first_day DATE, last_day DATE);
How to fix it: Just because the word alter is reserved does not mean it cannot be used, it just has special requirements to use it as the MySQL engine is trying to call the functionality for the alter command. To fix the issue, you will want to surround the word with backticks ``, this is usually the button just to the left of the "1" button on the keyboard. The code block below shows how the code will need to look in order to run properly.
CREATE TABLE `alter` (first_day DATE, last_day DATE);
Missing Data
Sometimes data in the database is missing. This can cause issues when this data is required for a query. For example, if a database is built requiring an ID number for every student, it is reasonable to assume a query will be built to pull a student record by that ID number. Such a query would look like:
SELECT * from students WHERE studentID = $id
If the $id is never properly filled in the code, the query would look like this to the server:
SELECT * from students WHERE studentID =
www.google.com.tr/
In MySQL, I'm trying to select rows from a table using a WHERE clause which is not fixed (known in advance), but instead is stored in another table and will be pulled from that other table at run-time.
Although seemingly similar questions already have been posted on Stackoverflow, there is none which exactly covers that problem.
What I'm trying to achieve is the following :
SELECT * FROM users WHERE (SELECT user_condition FROM user_condition_table WHERE id = '1')
The subquery
SELECT user_condition FROM user_condition_table WHERE id = '1'
Would produce the string
first_name like 'Br' and last_name like 'A%'
So the overall query would look like :
SELECT * FROM users WHERE first_name like 'Br' and last_name like 'A%'
I guess the problem here is that the returned string from select is returned as string and not as an SQL statement.
Is there any way to make that work?
You must use Dynamic SQL --> Prepare / Execute / Deallocate commands.
Dynamic SQL
Dynamic SQL is a programming technique that enables you to build SQL
statements dynamically at runtime. You can create more general
purpose, flexible applications by using dynamic SQL because the full
text of a SQL statement may be unknown at compilation.
Simple live example
DROP TABLE IF exists Users;
CREATE TABLE USERS(
first_name varchar(20),
last_name varchar(20)
);
INSERT INTO Users VALUES('Tom','Smile'),('Bob','Hamilton'),('Jane','Moon');
DROP TABLE IF exists user_condition_table;
CREATE TABLE user_condition_table(
id int,
user_condition Varchar(100)
);
INSERT INTO User_condition_table
VALUES(1, 'first_name like ''B%'' and last_name like ''H%''');
and the query:
SET #x = ( SELECT user_condition FROM user_condition_table WHERE id = '1');
SET #x = concat('SELECT * FROM USERS WHERE ', #x );
SELECT #x;
PREPARE stmt2 FROM #x;
EXECUTE stmt2;
DEALLOCATE PREPARE stmt2;
This is an interesting question. I have never tried it because I did such things in application code until now. Nevertheless, I got interested in the subject and just came up with the following solution:
SELECT #cond := user_condition FROM user_condition_table WHERE (id = 1);
SELECT #sqlstring := CONCAT('SELECT * FROM users WHERE ', #cond);
PREPARE sqlquery FROM #sqlstring;
EXECUTE sqlquery;
DEALLOCATE PREPARE sqlquery;
Some remarks:
It seems that you need to use session variables (denoted by the leading #).
This is not a stored procedure; it is just some normal SQL statements which you can type directly from within MySQL client, for example.
First I create my table with one text column in 5.6.34 - MySQL Community Server (GPL) like so :
CREATE TABLE `$_query_test`.`t1` ( c1 TEXT NOT NULL ) ENGINE = InnoDB;
Then I execute:
SET #var = 321; INSERT INTO `t1` (c1) VALUES (#var);
I get one entry with a value of 321 as expected, but when i execute:
SET #var = '_string_';
ALTER TABLE `t1` ADD #var TEXT;
I get an error
#1064 - 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 '#var TEXT' at line 1
What is the proper syntax to do something like this? I have searched far and wide and so far got nothing on this.
The answer is that you cannot substitute an sql variable in place of an identifier (e.g. field name, table name, etc). Period.
What you can do is to create the full sql statement as a string either on application level or in mysql with the identifier already substituted and you execute the full statement.
In mysql you can do this via prepared statements:
PREPARE stmt1 FROM CONCAT('ALTER TABLE `t1` ADD ', #var, ' TEXT');
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
You can use variables for values but not field names. You will need to do that with whatever server language you are using before composing the alter table.
I want to find all the tables in my db that contain the column name Foo, and update its value to 0, I was thinking something like this, but I don't know how to place the UPDATE on that code, I plan on having this statement on the Events inside the MySQL database, I'm using WAMP, the idea is basically having an event run daily which sets all my 'Foo' Columns to 0 without me having to do it manually
SELECT TABLE_NAME, COLUMN_NAME
FROM INFORMATION_SCHEMA.COLUMNS
WHERE column_name LIKE 'Foo'
No, not in a single statement.
To get the names of all that tables that contain column named Foo:
SELECT table_schema, table_name
FROM information_schema.columns
WHERE column_name = 'Foo'
Then, you'd need an UPDATE statement for each table. (It's possible to do update multiple tables in a single statement, but that would need to be an (unnecessary) cross join.) It's better to do each table separately.
You could use dynamic SQL to execute the UPDATE statements in a MySQL stored program (e.g. PROCEDURE)
DECLARE sql VARCHAR(2000);
SET sql = 'UPDATE db.tbl SET Foo = 0';
PREPARE stmt FROM sql;
EXECUTE stmt;
DEALLOCATE stmt;
If you declare a cursor for the select from information_schema.tables, you can use a cursor loop to process a dynamic UPDATE statement for each table_name returned.
DECLARE done TINYINT(1) DEFAULT FALSE;
DECLARE sql VARCHAR(2000);
DECLARE csr FOR
SELECT CONCAT('UPDATE `',c.table_schema,'`.`',c.table_name,'` SET `Foo` = 0') AS sql
FROM information_schema.columns c
WHERE c.column_name = 'Foo'
AND c.table_schema NOT IN ('mysql','information_schema','performance_schema');
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
OPEN csr;
do_foo: LOOP
FETCH csr INTO sql;
IF done THEN
LEAVE do_foo;
END IF;
PREPARE stmt FROM sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END LOOP do_foo;
CLOSE csr;
(This is just an rough outline of an example, not syntax checked or tested.)
FOLLOWUP
Some brief notes about some ideas that were probably glossed over in the answer above.
To get the names of the tables containing column Foo, we can run a query from the information_schema.columns table. (That's one of the tables provided in the MySQL information_schema database.)
Because we may have tables in multiple databases, the table_name is not sufficient to identify a table; we need to know what database the table is in. Rather than mucking with a "use db" statement before we run an UPDATE, we can just reference the table UPDATE db.mytable SET Foo....
We can use our query of information_schema.columns to go ahead and string together (concatenate) the parts we need to create for an UPDATE statement, and have the SELECT return the actual statements we'd need to run to update column Foo, basically this:
UPDATE `mydatabase`.`mytable` SET `Foo` = 0
But we want to substitute in the values from table_schema and table_name in place of mydatabase and mytable. If we run this SELECT
SELECT 'UPDATE `mydatabase`.`mytable` SET `Foo` = 0' AS sql
That returns us a single row, containing a single column (the column happens to be named sql, but name of the column isn't important to us). The value of the column will just be a string. But the string we get back happens to be (we hope) a SQL statement that we could run.
We'd get the same thing if we broke that string up into pieces, and used CONCAT to string them back together for us, e.g.
SELECT CONCAT('UPDATE `','mydatabase','`.`','mytable','` SET `Foo` = 0') AS sql
We can use that query as a model for the statement we want to run against information_schema.columns. We'll replace 'mydatabase' and 'mytable' with references to columns from the information_schema.columns table that give us the database and table_name.
SELECT CONCAT('UPDATE `',c.table_schema,'`.`',c.table_name,'` SET `Foo` = 0') AS sql
FROM information_schema.columns
WHERE c.column_name = 'Foo'
There are some databases we definitely do not want to update... mysql, information_schema, performance_schema. We either need whitelist the databases containing the table we want to update
AND c.table_schema IN ('mydatabase','anotherdatabase')
-or- we need to blacklist the databases we definitely do not want to update
AND c.table_schema NOT IN ('mysql','information_schema','performance_schema')
We can run that query (we could add an ORDER BY if we want the rows returned in a particular order) and what we get back is list containing the statements we want to run. If we saved that set of strings as a plain text file (excluding header row and extra formatting), adding a semicolon at the end of each line, we'd have a file we could execute from the mysql> command line client.
(If any of the above is confusing, let me know.)
The next part is a little more complicated. The rest of this deals with an alternative to saving the output from the SELECT as a plain text file, and executin the statements from the mysql command line client.
MySQL provides a facility/feature that allows us to execute basically any string as a SQL statement, in the context of a MySQL stored program (for example, a stored procedure. The feature we're going to use is called dynamic SQL.
To use dynamic SQL, we use the statements PREPARE, EXECUTE and DEALLOCATE PREPARE. (The deallocate isn't strictly necessary, MySQL will cleanup for us if we don't use it, but I think it's good practice to do it anyway.)
Again, dynamic SQL is available ONLY in the context of a MySQL stored program. To do this, we need to have a string containing the SQL statement we want to execute. As a simple example, let's say we had this:
DECLARE str VARCHAR(2000);
SET str = 'UPDATE mytable SET mycol = 0 WHERE mycol < 0';
To get the contents of str evaluated and executed as a SQL statement, the basic outline is:
PREPARE stmt FROM str;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
The next complicated part is putting that together with the query we are running to get string value we want to execute as SQL statements. To do that, we put together a cursor loop. The basic outline for that is to take our SELECT statement:
SELECT bah FROM humbug
And turn that into a cursor definition:
DECLARE mycursor FOR SELECT bah FROM humbug ;
What we want to is execute that and loop through the rows it returns. To execute the statement and prepare a resultset, we "open" the cursor
OPEN mycursor;
When we're finished with it, we're goin to issue a "close", to release the resultset, so the MySQL server knows we don't need it anymore, and can cleanup, and free up the resources allocated to that.
CLOSE mycursor;
But, before we close the cursor, we want to "loop" through the resultset, fetching each row, and do something with the row. The statement we use to get the next row from the resultset into a procedure variable is:
FETCH mycursor INTO some_variable;
Before we can fetch rows into variables, we need to define the variables, e.g.
DECLARE some_variable VARCHAR(2000);
Since our cursor (SELECT statement) is returning only a single column, we only need one variable. If we had more columns, we'd need a variable for each column.
Eventually, we'll have fetched the last row from the result set. When we attempt to fetch the next one, MySQL is going to throw an error.
Other programming languages would let us just do a while loop, and let us fetch the rows and exit the loop when we've processed them all. MySQL is more arcane. To do a loop:
mylabel: LOOP
-- do something
END LOOP mylabel;
That by itself makes for a very fine infinite loop, because that loop doesn't have an "exit". Fortunately, MySQL gives us the LEAVE statement as a way to exit a loop. We typically don't want to exit the loop the first time we enter it, so there's usually some conditional test we use to determine if we're done, and should exit the loop, or we're not done, and should go around the the loop again.
mylabel: LOOP
-- do something useful
IF some_condition THEN
LEAVE mylabel;
END IF;
END LOOP mylabel;
In our case, we want to loop through all of the rows in the resultset, so we're going to put a FETCH a the first statement inside the loop (the something useful we want to do).
To get a linkage between the error that MySQL throws when we attempt to fetch past the last row in the result set, and the conditional test we have to determine if we should leave...
MySQL provides a way for us to define a CONTINUE HANDLER (some statement we want performed) when the error is thrown...
DECLARE CONTINUE HANDLER FOR NOT FOUND
The action we want to perform is to set a variable to TRUE.
SET done = TRUE;
Before we can run the SET, we need to define the variable:
DECLARE done TINYINT(1) DEFAULT FALSE;
With that we, can change our LOOP to test whether the done variable is set to TRUE, as the exit condition, so our loop looks something like this:
mylabel: LOOP
FETCH mycursor INTO some_variable;
IF done THEN
LEAVE mylabel;
END IF;
-- do something with the row
END LOOP mylabel;
The "do something with the row" is where we want to take the contents of some_variable and do something useful with it. Our cursor is returning us a string that we want to execute as a SQL statement. And MySQL gives us the dynamic SQL feature we can use to do that.
NOTE: MySQL has rules about the order of the statements in the procedure. For example the DECLARE statement have to come at the beginning. And I think the CONTINUE HANDLER has to be the last thing declared.
Again: The cursor and dynamic SQL features are available ONLY in the context of a MySQL stored program, such as a stored procedure. The example I gave above was only the example of the body of a procedure.
To get this created as a stored procedure, it would need to be incorporated as part of something like this:
DELIMITER $$
DROP PROCEDURE IF EXISTS myproc $$
CREATE PROCEDURE myproc
NOT DETERMINISTIC
MODIFIES SQL DATA
BEGIN
-- procedure body goes here
END$$
DELIMITER ;
Hopefully, that explains the example I gave in a little more detail.
This should get all tables in your database and append each table with update column foo statement Copy and run it, the copy the output and run as sql
select concat('update ',table_name,' set foo=0;') from information_schema.tables
where table_schema = 'Your database name here' and table_type = 'base table';