I am using SQLAlchemy to describe my database content. I am using Metadata object, reflect method and looping over tables. (Code below)
When I am looping over the tables I need to distinguish which one of them is a table and which one is a view. I can't seem to find any attributes or method that could help me.
metadata = MetaData(bind=self.engine, schema=schema)
metadata.reflect(views=True)
for key, table in metadata.tables.items():
print(table.name)
# Here I need to know if table is a table or a view
I tried printing the type of the table object, but it's always type sqlalchemy.sql.schema.Table
Thanks!
I don't think that SQLAlchemy distinguishes between tables and views - both are Table instances. A workaround - at the cost of an extra query - might be to compare the results of reflecting with and without views:
tmeta = metadata.reflect(engine)
vmeta = metadata.reflect(engine, views=True)
view_names = vmeta.tables.keys() - t.meta.tables.keys()
Related
I'm interested in getting the structure of each table in my DB.
Currently I'm using: DESCRIBE TABLE table1.
However, this means I have to do a separate query for each table. Was wondering whether there is a query I can get the structure of several tables at once (and therefore saving me some queries)?
Thanks,
Nir.
You can use Account Usage/Information Schema view COLUMNS
https://docs.snowflake.com/en/sql-reference/account-usage/columns.html
Following article have a slight difference example of using COLUMNS view to create a select statement but it should give you an idea
https://community.snowflake.com/s/article/Select-columns-based-on-condition-in-Snowflake-using-Information-Schema-and-Stored-Procedure
You have a couple options:
you can use the COLUMNS view in the information schema
https://docs.snowflake.com/en/sql-reference/info-schema/columns.html
Note: The view only displays objects for which the current role for the session has been granted access privileges.
you can use the COLUMNS view in the account_usage share schema:
https://docs.snowflake.com/en/sql-reference/account-usage/columns.html
Note: this will show all the columns in all tables, will show deleted objects and such as well.
Also note, there is a delay in the data (latency could be as much as 90 minutes, typically isn't though)
I understand that a view is only a saved SQL query that can be considered as a virtual table. However, it seems to me that there is not much difference between creating a new table using the SELECT INTO statement and creating a view. What are some of the major distinctions between the two methods? Under what situations would you use one or the other?
Let's start with aknowledging that by what you say about views, you are not talking about indexed views. Because these are actually implemented with existing tables in the background. So, we are talking about non-indexed views.
The two methods are very different - in fact, they have nothing in common. It is strange, because you both mention:
"view = only saved sql query"
"into = creating new table"
Isn't this a contradiction? The result of select into is actually a table, a view is not.
Not sure why are you asking about this or what are you trying to accomplish. In my experience, I use select into to fastly create logically temporary tables that have the same columns with the original without having to type all columns. This method of creating tables is generally inferior to the create table command and a subsequent insert, because no indexes and other stuff can be made - thus its use in adhoc queries or as a temporary entity.
I use the Zend 2 Framework to build my web application. I implemented my database table models by this tutorial: http://framework.zend.com/manual/2.1/en/user-guide/database-and-models.html
I have a many-to-many relationship between two models in my database. To get data from them I googled and found this link: http://mattmccormick.ca/2010/04/24/how-to-easily-create-models-and-table-relationships-in-zend-framework/
The problem is that all the table models extends from Zend_Db_Table_Abstract in the example. I don't know how to get data from the models.
I have a table containing votings, every voting has a unique hash id. Every voting also has tags. Therefore I defined a table tags with all the tags available and a voting_tag_map where all many-to-many relationships are mapped.
What I have tried so far is the following, that's code from my VotingTable class:
public function getTagsByVoting($votingHash){
$select = $this->tableGateway->getSql()->select();
$select->from(array('v' => 'voting'))
->join('voting_tag_map', 'v.voting_id=voting_tag_map.voting_id')
->join('tags', 'voting_tag_map.tag_id=tags.tag_id');
$resultSet = $this->tableGateway->selectWith($select);
return $resultSet;
}
It says then:
Since this object was created with a table and/or schema in the constructor, it is read only.
Thats because of the from() method. If I delete the from() method, it says:
Statement could not be executed
Can anyone help me please?
Since this object was created with a table and/or schema in the constructor, it is read only.
This error is because you are trying to set the table name in the from clause, but it's already been set in the contructor of the TableGateway, and you can't change it once set.
If you really need to do this then you can extens AbstractTableGateway yourself then you won't have to add a string tablename to the contructor, but you don't really need to use an alias on your main table...
The SQL error you get when you comment out the from() method will be due to your referencing the votes table as it's alias 'v' in your join, when you are not using the alias v, try changing it to 'voting.XXX' from 'v.XXX'
The question is how database design should I apply for this situation:
main table:
ID | name | number_of_parameters | parameters
parameters table:
parameter | parameter | parameter
Number of elements in parameters table does not change. number_of_parameters cell defines how many parameters tables should be stored in next cell.
I have problems to move from object thinking to database design. So when we talk about object one row has as much parameters as number_of_parameters says.
I hope that description of requirements is clear. What is the correct way to design such database. If someone can provide some SQL statments to obtain it it would be nice. But the main goal of this question is to understand how to make such architecture.
I want to use SQLite to create this database.
The relational way is to have two tables. The main table has an ID, name and as many other universally-present parameters as possible. The parameters table contains a mapping from an ID in the main table to a parameter name and a parameter value; the main table ID should be a foreign key, and the combination of ID and name should be unique.
The number of parameters can be found by just counting the number of rows with a particular ID.
If you can serialize the data whiile saving to the database and deserialize it back when you get the record it will work. You can get total number of objects in serialized container and save the count to the number_of_parameters field and serialized data in parameters field.
There isn't one perfect correct way, but if you want to use a relational database, you preferably have relational tables.
If you have a key-value database, you place your serialized data as a document attached to your key.
If you want a hybrid solution, both human editable and single table, you can serialize your data to a human-readable format such as yaml, which sees heavy usage in configuration sections of open source projects.
I'm new to complex database design. I'm currently into a project where the user should be able to retrieve Instructions based on a combination of 18 columns. So my parameter Table has the following columns
Job
State
Manager
ProcessCode
ProcessType
(rest of the columns truncated).
InstructionID (FK of Instruction Table)
When adding / Modifying the instruction, he can choose multiple options in each of the above parameters. The Stored Procedure will store data in all combinations possible, in order facilitate easy retrieval, as during search (retrieval) only one option will be chosen in each of the columns.
There can be multiple instructions for same combination and the same instruction can apply to multiple combinations.
I have somehow created the SP for adding instruction, but am now struck with modification. When my Webpage passes the new combination to SP, what is the best way to update the Table?
I could delete all existing rows and create new rows for new combination, but I wanted to maintain the created date and created user columns. Further, there is a requirement to maintain history of these in a separate history table.
Sorry for the length of the question... And, Thank you for help.
If you're trying to retrieve data based on a combination of parameters then you can set the parameters to have the default value of NULL e.g.
CREATE PROC spProcName
#FieldName INT = NULL
The only other thing to do is set the WHERE section of the statement to look at the parameter values and compare them to see if they or null or not e.g.
WHERE ((FieldName = #FieldName) OR (#FieldName IS NULL))
Use this for querying the tables and use standard update queries in a similar fashion using the default parameter value of null but setting the value like this:
FieldName = ISNULL(#FieldName, FieldName)
Which lets you update only given parameters.
Hope this is something you are after, I can give a full example if needed.
What you have is many-to-many relationship, so I would suggest you use: