I need to store select statement results (multiple rows and Columns) in a variable to use later, Is there a way to do it in mySql only. I want to use it in stored procedure.
I know that single result of a select statement can easily be stored in a variable, but is there any to store multiple rows and columns?
If there is. Then What is that and its Ok.
If not then how a scenario like following is solvable in mySql? I have written following ideal/desired code supposing there exists a dataTbale.
Declare dt DataTable;
set dt = select column_name,table_name from information_schema.columns
where table_schema='emp';
DECLARE i INT; DECLARE c INT; set i=0; set c=dt.Rows.Count;
WHILE i < c
#q = concat ('select ', dt.Rows[i][0],' from ',dt.Rows[i][1]);
prepare s1 from #q;
execute s1;deallocate prepare s1;
END WHILE;
Each Row and its two cells of dt are used in while. Is there any facility of two-D array or some other?
What is being tried to do? Above code in while is intended to display all values from all columns (1 by one) from all tables of a database
What is question? Title and 1st Statement of Question.
Instead of storing all the results at once, use a cursor: http://dev.mysql.com/doc/refman/5.0/en/cursors.html
Then loop while reading the cursor.
Related
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 have this strange problem. I have a table containing 2 numbers, but a simple calculation makes a precision fault:
mysql> create table tblX as select '2.7' as H, '2100' as H2;
Query OK, 1 row affected (0.32 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select *, h-h2/1000 from tblX;
+-----+------+--------------------+
| H | H2 | h-h2/1000 |
+-----+------+--------------------+
| 2.7 | 2100 | 0.6000000000000001 |
+-----+------+--------------------+
1 row in set (0.00 sec)
This example recreates a problem I have in a setup, where formulas, field and their values are extracted in a way where some fields have to be varchars but others are used in calculations as above. So the formula can not be changed. Another example could be a formula like: if(code='c' and h-h2/1000>0.6,h,0)
where the fields are created by a pivot operation.
My assumption is, that implicit conversion of varchar to decimal causes this problem. My question is: Are there any server settings that could change this implicit conversion. Or are there any other solution for this.
EDIT:
If I modify the columns like this:
alter table tblX MODIFY column h decimal(20,4), MODIFY h2 decimal(20,4);
The result will be correct. But I do not know the names of the columns in advance as they are created on the fly in temporary tables. It would be nice if it was possible to create a script that modify all columns that is only containing numeric values.
I am not aware about any server settings. But you can do explicit conversion in the sql as well.
Try something like this :
select *, CAST(h-h2/1000 AS DECIMAL(10,2)) from tblX;
Refer MySQL documentation. https://dev.mysql.com/doc/refman/5.5/en/fixed-point-types.html
The problem is, that varchars in a formula apparently implicit converts to floats and not decimals. Changing the formulas is not an option in this case. But modifying column datatype is an option.
The solution is to make a stored procedure that loops thru the relevant columns and modify them to a decimal datatype.
The table that contains the incorrect datatypes is named tmp.
I made a temporary table called tmpPar containing all column names that take part in a formula as numeric values. Then I set up a cursor and loop thru all those columns. Cursor loop looks like this:
DECLARE a varchar(255);
DECLARE cur1 CURSOR FOR SELECT CName FROM tmpPar;
...
Open cur1;
simple_loop: LOOP
fetch cur1 into a;
IF no_more_rows=1 THEN
LEAVE simple_loop;
END IF;
set #sql = null;
set #sql = concat('alter table tmp MODIFY column ' , a , ' decimal(20,4)');
PREPARE stmt FROM #sql;
EXECUTE stmt;
END LOOP simple_loop;
Close cur1;
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';
I am trying to update a MySQL InnoDB table with c. 100 million rows. The query takes close to an hour, which is not a problem.
However, I'd like to split this update into smaller chunks in order not to block table access. This update does not have to be an isolated transaction.
At the same time, the splitting of the update should not be too expensive in terms of additional overhead.
I considered looping through the table in a procedure using :
UPDATE TABLENAME SET NEWVAR=<expression> LIMIT batchsize, offset,
But UPDATE does not have an offset option in MySQL.
I understand I could try to UPDATE ranges of data that are SELECTed on a key, together with the LIMIT option, but that seems rather complicated for that simple task.
I ended up with the procedure listed below. It works but I am not sure whether it is efficient with all the queries to identify consecutive ranges. It can be called with the following arguments (example):
call chunkUpdate('SET var=0','someTable','theKey',500000);
Basically, the first argument is the update command (e.g. something like "set x = ..."), followed by the mysql table name, followed by a numeric (integer) key that has to be unique, followed by the size of the chunks to be processed. The key should have an index for reasonable performance. The "n" variable and the "select" statements in the code below can be removed and are only for debugging.
delimiter //
CREATE PROCEDURE chunkUpdate (IN cmd VARCHAR(255), IN tab VARCHAR(255), IN ky VARCHAR(255),IN sz INT)
BEGIN
SET #sqlgetmin = CONCAT("SELECT MIN(",ky,")-1 INTO #minkey FROM ",tab);
SET #sqlgetmax = CONCAT("SELECT MAX(",ky,") INTO #maxkey FROM ( SELECT ",ky," FROM ",tab," WHERE ",ky,">#minkey ORDER BY ",ky," LIMIT ",sz,") AS TMP");
SET #sqlstatement = CONCAT("UPDATE ",tab," ",cmd," WHERE ",ky,">#minkey AND ",ky,"<=#maxkey");
SET #n=1;
PREPARE getmin from #sqlgetmin;
PREPARE getmax from #sqlgetmax;
PREPARE statement from #sqlstatement;
EXECUTE getmin;
REPEAT
EXECUTE getmax;
SELECT cmd,#n AS step, #minkey AS min, #maxkey AS max;
EXECUTE statement;
set #minkey=#maxkey;
set #n=#n+1;
UNTIL #maxkey IS NULL
END REPEAT;
select CONCAT(cmd, " EXECUTED IN ",#n," STEPS") AS MESSAGE;
END//
I need to perform the same procedure for several tables in my DB. The poblem is that that procedure contains the following line:
DECLARE tableIt CURSOR FOR select id from table where column=inputParam ;
table is the table the procedure works with. And I can't find a way to make that table name to be dynamic, i.e. to read it from an input parameter.
Right now I have 8 different procedures (one for each table) which differentiate from each other only by one word (the name of the table).
That is really a pain since I have to make every change to the procedure 8 times.
Is it possible to parameterize the select statement for the CURSOR so I can have only one procedure??
Dynamic cursors does not seem to be supported in Mysql.
http://dev.mysql.com/doc/refman/5.1/en/cursors.html
You can work around it
http://forums.mysql.com/read.php?98,133197,149099#msg-149099
"DROP VIEW IF EXISTS v1;
SET #stmt_text=CONCAT("CREATE VIEW v1 AS SELECT c_text FROM ", t_name);
PREPARE stmt FROM #stmt_text;
EXECUTE stmt;
BEGIN
DECLARE v_text VARCHAR(45);
DECLARE done INT DEFAULT 0;
DECLARE c cursor FOR SELECT c_text FROM v1;"