I am new SQL and I was wondering if there is any quick way of getting a global "view" of a new database (if for example you are starting to use a database you know nothing about and you want to just get a global idea of how the entire database looks like).
In other words is there a way to :
Maybe get some graphical representation of the database? - a sort of diagram that shows the relation between all tables
Maybe do some sort of query that could return the no. of rows, no. columns (and ideally column names) of each table in the database?
Apologies if this is a really basic question, I am very new to SQL. I am currently using PostgreSQL and PgAdmin4. Thanks
Related
What's the best approch to have a dynamic query like
select $dynamic_columns from table
But also prevent error like column not found and get result with available columns. considering $dynamic_columns is given by end users.
One approach would be to store the schema in java object and filter it. Again if schema is update in DB we will need to update the schema java object cache. is there any better way to handle this?
Be careful with this as it is more vulnerable to SQL injection.
Never let the user type something into a text field, instead build a
list for them to select from.
For building the list, I think the best approach is to use the JDBC method DatabaseMetaData.getColumns(...) to retrieve a list of columns for a table. I don't think there's a need to cache anything.
I am starting to learn Spring and faced with some issues regarding spring-jdbc.
First, I tried run the example from this: https://spring.io/guides/gs/relational-data-access/ and it worked. Then, I commented lines with droping and creating new tables(http://pastebin.com/zcJHsL1P), in order to not override data, but just get it from db and show it. However, spring showed me error:
Table "CUSTOMERS" not found; SQL statement: ...
So, my question is: What should I do to store my database permanently? I don't want to recreate all time new database, I want create it once and update it.
P.S. I used H2 database. Maybe problem exists in tis db?
That piece of code looks like you are "prototyping" something; so it's easier to automatically create a new database (schema, tables, data) on the fly, execute and/or test whatever you want to...and finish the execution.
If you want to persist your data and only modify/update it, either use H2 with the "file layout" or use MySQL, PostreSQL, etcetera.
By the way, the reason you are getting Table "CUSTOMERS" not found; SQL statement: ... is because you are using H2 as an in-memory database and every time you start your application you need to re-create the tables and populate them with data.
In answer to this question I've learned that you can create empty table in PostgreSQL.
create table t();
Is there any real use case for this? Why would you create empty table? Because you don't know what columns it will have?
These are the things from my point of view that a column less table is good for. They probably fall more into the warm and fuzzy category.
1.
One practical use of creating a table before you add any user
defined columns to it is that it allows you to iterate fast when
creating a new system or just doing rapid dev iterations in general.
2.
Kind of more of 1, but lets you stub out tables that your app logic or procedure can make reference too, even if the columns have
yet to
be put in place.
3.
I could see it coming in handing in a case where your at a big company with lots of developers. Maybe you want to reserve a name
months in advance before your work is complete. Just add the new
column-less table to the build. Of course they could still high
jack it, but you may be able to win the argument that you had it in
use well before they came along with their other plans. Kind of
fringe, but a valid benefit.
All of these are handy and I miss them when I'm not working in PostgreSQL.
I don't know the precise reason for its inclusion in PostgreSQL, but a zero-column table - or rather a zero-attribute relation - plays a role in the theory of relational algebra, on which SQL is (broadly) based.
Specifically, a zero-attribute relation with no tuples (in SQL terms, a table with no columns and no rows) is the relational equivalent of zero or false, while a relation with no attributes but one tuple (SQL: no columns, but one row, which isn't possible in PostgreSQL as far as I know) is true or one. Hugh Darwen, an outspoken advocate of relational theory and critic of SQL, dubbed these "Table Dum" and "Table Dee", respectively.
In normal algebra x + 0 == x and x * 0 == 0, whereas x * 1 == x; the idea is that in relational algebra, Table Dum and Table Dee can be used as similar primitives for joins, unions, etc.
PostgreSQL internally refers to tables (as well as views and sequences) as "relations", so although it is geared around implementing SQL, which isn't defined by this kind of pure relation algebra, there may be elements of that in its design or history.
It is not empty table - only empty result. PostgreSQL rows contains some invisible (in default) columns. I am not sure, but it can be artifact from dark age, when Postgres was Objected Relational database - and PG supported language POSTQUEL. This empty table can work as abstract ancestor in class hierarchy.
List of system columns
I don't think mine is the intended usage however recently I've used an empty table as a lock for a view which I create and change dynamically with EXECUTE. The function which creates/replace the view has ACCESS EXCLUSIVE on the empty table and the other functions which uses the view has ACCESS.
I am currently experimenting with Tableau Extract API to generate some TDE from the tables I have in a PostgreSQL database. I was able to write a code to generate the TDE from single table, but I would like to do this for multiple joined tables. To be more specific, if I have two tables that are inner joined by some field, how would I generate the TDE for this?
I can see that if I am working with small number of tables, I could use a SQL query with JOIN clauses to create a one gigantic table, and generate the TDE from that table.
>> SELECT * FROM table_1 INNER JOIN table_2
INTO new_table_1
ON table_1.id_1 = table_2.id_2;
>> SELECT * FROM new_table_1 INNER JOIN TABLE_3
INTO new_table_2
ON new_table_1.id_1 = table_3.id_3
and then generate the TDE from new_table_2.
However, I have some tables that have over 40 different fields, so this could get messy.
Is this even a possibility with current version of the API?
You can read from as many tables or other sources as you want. Or use complex query with lots of joins, or create a view and read from that. Usually, creating a view is helpful when you have a complex query joining many tables.
The data extract API is totally agnostic about how or where you get the data to feed it -- the whole point is to allow you to grab data from unusual sources that don't have pre-built drivers for Tableau.
Since Tableau has a Postgres driver and can read from it directly, you don't need to write a program with the data extract API at all. You can define your extract with Tableau Desktop. If you need to schedule automated refreshes of the extract, you can use Tableau Server or its tabcmd command.
Many thanks for your replies. I am aware that I could use Tableau Desktop to define my extract. In fact, I have done this many times before. I am just trying to create the extracts using the API, because I need to create some calculated fields, which is near impossible to create using the Tableau Desktop.
At this point, I am hesitant to use JOINs in the SQL query because the resulting table would look too complicated to comprehend (some of these tables also have same field names).
When you say that I could read from multiple tables or sources, does that mean with the Tableau Extract API? At this point, I cannot find anywhere in this API that accommodates multiple sources. For example, I know that when I use multiple tables in the Tableau Desktop, there are icons on the left hand side that tells me that the extract is composed of multiple tables. This just doesn't seem to be happening with the API, which leaves me stranded. Anyways, thank you again for your replies.
Going back to the topic, this is something that I tried few days ago on my python code
try:
tdefile= tde.Extract("extract.tde")
except:
os.remove("extract.tde")
tdefile = tde.Extract("extract.tde")
tableDef = tde.TableDefinition()
# Read each column in table and set the column data types using tableDef.addColumn
# Some code goes here...
for eachTable in tableNames:
tableAdd = tdeFile.addTable(eachTable, tableDef)
# Use SQL query to retrieve bunch_of_rows from eachTable
for some_row in bunch_of_rows:
# Read each row in table, and set the values in each column position of each row
# Some code goes here...
tableAdd.insert(some_row)
some_row.close()
tdefile.close()
When I execute this code, I get the error that eachTable has to be called "Extract".
Of course, this code has its flaws, as there is no where in this code that tells how each table are being joined.
So I am little thrown off here, because it doesn't seem like I can use multiple tables unless I use JOINs to generate one table that contains everything.
The question pretty much sums it up. I've got to replace text in a large number for store procedures. Its not so many that doing it manually is impossible, but enough that I'm asking the question. I also prefer automation as it reduces the change of user error when we make the change in production.
I can Identify them like this:
select OBJECT_DEFINITION(object_id), *
from sys.procedures
where OBJECT_DEFINITION(object_id) like '%''MyExampleLiteral''%'
order by name
Is there any way to mass update them all to change 'MyExampleLiteral' to 'MyOtherExampleLiteral'?
I'd even settle for a way to open all the stored procs. Just Finding these store procs in a larger list will take some time.
I thought about generating alter statements using the above select statements, but then I lose line breaks.
Thanks in advance,
This is a Microsoft SQL Server.
There are different tools to use depending on the database in question. For example, Microsoft SQL Server Data Tools integrates with Visual Studio, and allows you to do these types of operations fairly easily. The database is stored in your solution as scripts, which you can then search and replace any keyword you wish. I'm assuming there would be similar tools available for other platforms.
You could do this with dynamic sql. Query the system tables to get all the SPs containing your "MyExampleLiteral":
SELECT [object_id] FROM sys.objects o
WHERE type_desc = 'SQL_STORED_PROCEDURE'
AND is_ms_shipped = 0
AND OBJECT_DEFINITION(o.[object_id]) LIKE '%<search string>%'
Then, write a while loop to go through those object_ids. In the while loop, get the OBJECT_DEFINITION() into a string and replace the "MyExampleLiteral", then replace CREATE PROCEDURE with ALTER PROCEDURE and execute the string using sp_executesql.
Doing something this crazy, make sure you backup the database first.