I know that I can view the properties and do some amazing things to learn my db/table/field structure using INFORMATION_SCHEMA.
However, is it possible to update a field value in for example the COLUMNS table and thus update the actual column? For example setting nullable from NO to YES.
If this is not directly possible, I realize that I could use the query to CONCAT an ALTER string and then run those strings. However is there in that case a way to instead run an eval() command to do this in one operation? Thanks.
To learn INFORMATION_SCHEMA: I'd start from reading documentation - INFORMATION_SCHEMA Tables. Then I'd try to edit objects and see what happens in these tables.
INFORMATION_SCHEMA tables are system tables, they cannot be modified. ALTER TABLE is the only way to change a table.
It was a beautiful idea, but unfortunately, it won't work.
The tables in INFORMATION_SCHEMA are read-only views, and are not actually tables. So there aren't any files or directories associated with them. You can only read their contents and can't run INSERT, UPDATE, or DELETE operations on them.
See https://dev.mysql.com/doc/refman/5.7/en/information-schema-introduction.html#information-schema-usage-notes for the lowdown.
And FYI, there's a Database Administrators Stack Exchange site if you want more targeted answers to database questions.
Related
TRIGGERs can be used to log changes to individual DB columns as described at https://stackoverflow.com/a/779250/569976 but that technique requires you have an IF statement for each column. It's not a huge issue if you're just interested in changes to one column BUT if you're interested in changes to all columns it becomes a bit more unweildy.
I can get all the column names of a table, dynamically, by querying the INFORMATION_SCHEMA.COLUMNS table. My question is... can I use that to dynamically reference the column names? Like in the TRIGGER you'd do OLD.columnName <> NEW.columnName but I don't think you can really make a column name dynamic like that.
In PHP you could use variable variables. eg. $obj->$var. But if MySQL has anything remotely similar that'd be news to me.
Any ideas? Or am I just going to go with the old fashioned approach of writing an IF statement for each of the 100s of columns this table has?
The trigger can only reference identifiers directly. You can't use a variable or an expression to name an identifier.
That would require dynamic SQL with PREPARE and EXECUTE so you could have the statement parsed at runtime from a string, but you can't PREPARE a new statement inside a trigger, because the trigger is already executing in the context of the currently executing statement.
The simplest solution is to write a trigger that references each column directly, with as many IF statements as there are columns in the table (I wonder why you have hundreds of columns in your table; that sounds like a different problem of bad design).
The comments above mention a binary log parser. Debezium is an example of an open-source binlog parser.
MySQL also supports an audit plugin architecture, but frankly the existing implementations of audit plugins are pretty clumsy.
https://www.mysql.com/products/enterprise/audit.html
https://mariadb.com/resources/blog/introducing-the-mariadb-audit-plugin/
https://github.com/mcafee/mysql-audit
I have a table in MySql in one server and a table in PostgreSQL in another server.
I want use use JOIN operation with those tables.
Is there any way to join the columns?
If not, is there any way to print the rows in same order?
Please help!!
Use mysql_fdw to define the MySQL table as a foreign table in PostgreSQL. Then you can join it with the PostgreSQL table in PostgreSQL.
You can use Materialize to achieve this.
Here is a sample demo project that you can run to see this in action:
You can find the code for the demo project and how to run it on GitHub here:
https://github.com/bobbyiliev/materialize-tutorials/tree/main/mz-join-mysql-and-postgresql
Hope this reference helps.
Yes, it is possible to work with multiple databases at the same time but you're looking in the wrong place. psycopg2 is just a library that simplifies accessing and manipulating data coming out of PostgreSQL but it doesn't go far beyond what you can do with psql. What you're looking to do you can solve on the database level by using Foreign Data Wrappers.
This does become more complicated in your schema definition but brings remote tables from host some.other.server database remote_db to appear as though they live on localhost in database local_db....
More:
https://dba.stackexchange.com/a/120682/197899
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 MySQL database preinstalled. I don't wont modify it. Does exist a built in method to check which and when a field has been modified in a specific table?
No, there's no built in method that does this in MySQL.
If you want this type of operation performed in the database, you would need to roll your own solution; and that would require you to modify the database, by adding tables and triggers to audit changes, for example.
i have to build an application to manage an existing MySQL database. This database was created with MySQLworkbench and some useful comments were added to its tables and columns.
I think it would be great to somehow, query that comments and show them to the user to explain "what that field is". The problem is i don't know if its possible to retrieve that comments (they are only visible from the workbench).
EDIT:
in the MySQL existing database i have to work with there is no INFORMATION_SCHEMA table. I think is something usual to find it but in my model there is no :S
Try with:
SELECT column_comment FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME='table_name';
You can parse the output of
show create table `YOURTABLE`;