I have to create a particular system of reviews in which I should do a lot of AVG in my query, so I want to ask if it's possible to create a table in SQL in which I have these Averages automatically updated?
Mysql 5.5.24-0ubuntu0.12.04.1
Yes, there are two ways to do this:
You can create a "view", which is like a table, except that its records come from a SQL query (usually derived from other tables) rather than being stored directly. See http://dev.mysql.com/doc/refman/5.6/en/create-view.html.
You can create a "trigger", which is procedural code that runs whenever a specified table is modified in a specified way. In your case, you would create a trigger that runs whenever one table is modified, and that re-generates the secondary table. See http://dev.mysql.com/doc/refman/5.6/en/create-trigger.html.
Related
Is there any way to detect when an ALTER TABLE statement is executed in MySQL? For example, if the following statement were executed on some_table, is there any way to detect that the column name changed from column_name_a to column_name_b and log it in another table in the DB?
ALTER TABLE `some_table`
CHANGE COLUMN `column_name_a` `column_name_b` VARCHAR(255) NULL DEFAULT NULL;
Thanks.
To my knowledge it is unfortunately not possible to put triggers on the INFORMATION_SCHEMA tables, since they are strictly spoken views and triggers can't be made to work on views. If triggers would be possible on the INFORMATION SCHEMA, then you could have a trigger on updates of the INFORMATION_SCHEMA.COLUMNS table to identify name changes.
However, what you can do is one of the following things:
option 1) Maintain a real table with all column names. Then create a function that checks for a discrepancy between the INFORMATION_SCHEMA.COLUMNS table abd your table. If there is one, you know the name has changed. You need to copy over the new name to your column name table and do whatever else you wanted to do upon name change.
The function to check for discrepancies then must be run periodically via the mysql scheduler in order to detect name changes as quickly as possible. Note that this is not a real time solution. There will be a lag between the ÀLTER TABLE command and its detection. If this is unacceptable in your scenario you need to go with
option 2) Do not call ÀLTER TABLE directly, but wrap it in a function. Within this function you can also call other functions to achieve what you need to achieve. If may be worth while to formulate the needed steps in a higher programming language that you use to drive your application. If this is not possible, you will be limited to the possibilities that are offered in functions/procedures in the mysql environment.
Sorry to not have a simpler way of doing this for you.
I have a simple data flow in SSIS (defined in visual studio 2013), which uses SQL to extract data from one sql server instance's table A to then add it to another SQL server instance's table B.
What is the best practice pattern to truncate the data in table B? A truncate statement like this:
TRUNCATE TABLE B
after the select statement for table A - especially when you have have a fairly big table to 'transmit'?
One thing I have done in cases like that is to create two copies of the same table and then a view that points to one or the other that has the name of the current table.
The SSIS package then determines which table is in use and sets the connection for the table to populate to the other table.
Then an exec SQl task truncates the table not currently in use. You may also want to drop any indexes at this point.
Then a dataflow populates the table not currently in use.
Then recreate any indexes you dropped.
Finally an exec SQL task drops and creates the view to use the table you just populated instead of the other one.
Total down time of the table being referenced? Generally less than a second for the drop and create view no matter how long it takes to populate the table.
I need to put all the result of a query to a temporary table then select from it, but due to the nature of the temporary table, I cannot refer to it more than once in the same query. So is it ok to create a uniquely named (regular) table for this and drop it after the query is completed? Or is there a better way to do this?
I tried using Derived tables but I cannot access it from different blocks.
It's not necessarily a matter of whether it's good practice or not. Whether it's a regular table, temporary table or derived table they all serve a purpose when trying to accomplish a task. Therefore, creating a regular table or a temporary table all depends on you and what you want your application to accomplish at the end of the day.
But based on your problem it seems that you might need to create a regular table in this situation since you require access to the table more than once, and you can drop the table after you're finished with it.
Is there a way to do this?
In case the DBMS command history got cleaned or, in my case, when many ALTER TABLE were used in the course of time.
I'm using MySQL.
Yes, it is as simple as
SHOW CREATE TABLE yourtable;
This will include all the subsequent ALTER TABLE statements. You cannot retrieve the table's original state.
Here is the relevant documentation
What is the purpose of a temporary table like in the following statement? How is it different than a regular table?
CREATE TEMPORARY TABLE tmptable
SELECT A.* FROM batchinfo_2009 AS A, calibration_2009 AS B
WHERE A.reporttime LIKE '%2010%'
AND A.rowid = B.rowid;
Temp tables are kept only for the duration of your session with the sever. Once the connection's severed for any reason, the table's automatically dropped. They're also only visible to the current user, so multiple users can use the same temporary table name without conflict.
Temporary table ceases to exist when connection is closed. So, its purpose is for instance to hold temporary result set that has to be worked on, before it will be used.
Temporary tables are mostly used to store query results that need further processing, for instance if the result needs to be queried or refined again or is going to be used at different occasions by your application. Usually the data stored in a temporary database contains information from several regular tables (like in your example).
Temporary tables are deleted automatically when the current database session is terminated.
Support for temporary tables exists to allow procedural paradigms in a set-based 4GL, either because the coder has not switched their 3GL mindset to the new paradigm or to work around a performance or syntax issue (perceived or otherwise).