I am having a weird problem with mysql. I am developing a visual application on C# that stores data into a database. Previously I used SQL for the Database, but my client changed his mind for mysql. So I recreated the same schema on mysql. Now it is happening a really odd thing: My tables are completely empty, but when I execute the views, they return me back data from the old SQL tables, when I read directly from the tables they appear empty. The user that I use to connect are different and the most strange thing is that it happens even when I execute the view on mysql workbench. I have even truncated the tables in mysql and still the same thing. Does anybody know what may cause this anomaly and how to solve it?
p.s. Workbench version 6.2; Sql version SQL SERVER 2014
Regards.
In MySQL, a view is a virtual table based on the result-set of an SQL statement.
It contains rows and columns, just like a real table in your database. The fields in a view are fields from one or more real tables in the database.
When you execute the views, they return back data from the old SQL tables. It is because your view still contains the data you run a while ago. You have forgotten to Drop your View every time you execute it. To Drop a MySQL view, try this one:
DROP VIEW view_name
Views do NOT contain data of any kind -- except for Materialized Views and MySQL does not have those. If views had to be dropped and recreated every time a DML statement was executed on a table, views would be utterly useless.
The only time a view can return old data is when one process changes the contents of a table used in the view and the view is queried by another process before the first process commits the changes. You have not specified how the tables are being changed and how they are being queried. Nor have you included the create view statement. You could well be using other tables than what you think. This can happen during initial design of a database if tables are being slapped around like mad.
Related
I have access to a remote database, and I would like to dump the schema and data of several views onto my local machine and load this into my local database as tables in a quick and easy way.
I lack the user privileges to run CREATE TABLE AS (SELECT * FROM target_view), otherwise this would be trivial to solve. In other words, I want to retrieve and recreate the "composite" schema of target_view as if it were a table.
I do not want the output of SHOW CREATE VIEW, as this only shows a complex SELECT statement with joins to various tables on remote I have limited ability to access. And a problem I'm seeing in MySQL 8.x is when I run SHOW CREATE TABLE on the view, this command simply acts as an alias of SHOW CREATE VIEW (which is reasonable).
Frustratingly, I can run DESCRIBE and see the schema of these views as they were tables. I really just need to convert this information into a CREATE TABLE statement without actually being able to run CREATE TABLE.
In case it weren't obvious, the key is to avoid manual reconstruction of these views' tabular schemas (as they may change in the future). I also want to avoid the solution of reverse engineering a generic table construction of 20-30 generic VARCHAR or TEXT columns from a CSV dump.
I don't know of any way to display the metadata of a result set in CREATE TABLE syntax.
What I would do given your circumstance is first create on your local MySQL instance the base table and the view, then you can use the CREATE TABLE AS SELECT ... syntax to produce a concrete table to match the metadata of the view result set.
I am trying to match two MySQL Queries (for now, the target is "Create VIEW") to analyze if the result of execution would result in the same effect to Database.
The source of the queries is not the same, making the syntax across the queries inconsistent.
To further simplify the question, let me add more details:
Let's say there is an already existing View in the database.
This View was created using a Create VIEW ... SQL statement.
There is a possibility that the Create VIEW ... statement get's updated, hence to reflect the changes in the database currently this statement is executed at the time of migration.
But, I want to avoid this situation, if the statement Create VIEW ... will result in the same structure as of the existing View in the database, I want to avoid executing it.
To generate the CREATE VIEW from database I am using SHOW CREATE VIEW... (comparing this with the query originally used to create the VIEW).
The primary restriction is I need to make this decision only at the time of migration and cannot presume any conclusions (say, using git diff or commit history...).
I have already done some search to look for a solution for this:
Found no direct solution for this problem (like a SQL engine to which I can feed both queries and know if the result would be the same).
Decided to Parse the queries and to achieve that ended up looking into ANTLR (also used by MYSQL WorkBench)
ANTLR's approach looks promising but, this will require an extensive rule-based parsing and creating a query match program from scratch.
I realized that just parsing queries is not enough, I have to create my own POJOs to store the atomic lexers from queries and then compare the queries based on some rules.
Even if I could find predefined POJOs, that would allow to quickly create a solution for this problem.
I have three identical tables, one on MySQL, one linked to this one on Access by ODBC, and a native in the same Access database.
When I update the table on MySQL, the linked table on Access updates, and vice versa. But I would like to know if it is possible that the linked table updates the native table (and vice versa)?
Access table
MySQL table
It really depends on how the local Access table is being updated. If it is ALWAYS updated say by a few forms, then you could add a after update even to those few forms, and put in code to update the MySQL table.
Another approch (again you only/always update the local tables) is to add a table trigger to the local table. In this table code event, you can actually have it call some VBA code, and that VBA code could then update/insert to the linked MySQL table. Once again, then the two tables will automatic remain in sync.
The other possible would be to add a time + date stamp column to the tables (both on MySQL side, and on the Access side). You could then write some VBA code to sync up the tables. Such code is not too hard, but in a multi-user setting, this can become quite a challenge, since while you are syncing the data, other users might also update the MySQL tables and thus your sync routines might well miss some tables. Database sync software and this subject can fill a few books the size of medical texts, and is a VERY complex subject.
However, why not just always use linked tables to MySQL, and be done with any requirements to sync data? Access makes a great client to SQL server or MySQL. If you eliminate the local tables, then you eliminate the need to sync your data.
I would like to recreate a few existing Access queries in a new database so that I can tweak them a bit. The problem I am running into is that some of the tables being queried do not seem to exist in the current database. These tables all end with a 1.
For example, INV_MTL_ITEM_LOCATIONS is an imported table in the database, but
INV_MTL_ITEM_LOCATIONS_1 is being queried even though it does not show up in the tables panel on the left.
Is this some type of duplication functionality that I am not aware of? The query runs without any errors.
No, the query runs on an aliased table. It's actually just querying INV_MTL_ITEM_LOCATIONS
Using SQL, you can create an alias for a table. This is especially useful when querying the same table twice in one query, but also commonly used to shorten queries.
Your query will probably look something like this:
SELECT something
FROM INV_MTL_ITEM_LOCATIONS AS INV_MTL_ITEM_LOCATIONS_1
Access automatically creates these aliases when using the query builder and if you add the same table more than once. When removing the non-aliased table, the other one stays aliased.
This is entirely normal, and as far as I know, never a problem.
Erik's answer explains it perfectly.
But to be exhaustive, you CAN actually create a query on a table that is NOT in the current database (nor a linked table).
Here is an example:
SELECT *
FROM History IN 'c:\test\mySecretBackend.accdb'
You can also create that in design view, just by
setting the query's Source Database property to c:\test\mySecretBackend.accdb
click on Show table
I have a production database in which I have multiple tables. There is one table in which I store the server responses for the API calls which I receive.
Now that particular table is not working properly. I mean, the data is getting added into that table.
When I click on info button of the table, am getting the details which include number of rows and all. I can see that it's increasing. But the problem is that, whenever I try to execute any query on that table, my MySql workbench crashes.
I tried repair table indoor.ServerResponse; but again this query also doesn't work. It keeps on running and somewhere down the line, it freezes.
I just have few thousand rows in that table.
Now my question is :
1) How do I repair the table ? Did I lose it all?
2) How do I make sure that this doesn't happen in future? As this is the prod database.
3) I am also not able to Alter other tables in the database. But the queries run fine. Why is it so?
P.S. Am using MySql workbench.