I have a generic table in global area and i want to use it in SELECT from. Is this possible or is there a way do this ?
Example Code:
FIELD-SYMBOLS: <gt_data> TYPE STANDARD TABLE.
CLASS-DATA: mo_data TYPE REF TO data.
CREATE DATA mo_data LIKE lt_data.
ASSIGN mo_data->* TO <gt_data>.
<gt_data> = lt_data.
SELECT data~matnr,
mbew~malzeme_deger
FROM zmm_ddl_mbew AS mbew
INNER JOIN #<gt_data> AS data ON data~matnr EQ mbew~matnr
INTO TABLE #DATA(lt_mbew).
If the Generic table you are asking about is an internal Table which the code snippet suggests, then
No i dont think you cant build a join to work on 2 different sources.
Unless there are some new kernel developments, the select statements are converted to DB SQL statements.
ABAP 7.5 documentation of Select statement refers to the from "data_source" as dbtab,View or cds_entity as possible sources.
Even if it was possible there are still other generic options that may make more sense. If the source internal data is small enough, then you can build a generic where clause to solve the problem.
Select from DBTAB where (string_cond).
If the size of the internal table is so large that you end up with half the data in memory and half on a DB, there may be a better generic solution anyway.
No, it is not possible. From the SELECT datasource help:
If the FROM clause is specified statically, the internal table cannot be a generically typed formal parameter or a generically typed field symbol. Objects like this can only be specified in a dynamic FROM clause and must represent a matching internal table at runtime
The above rule remains valid whether itab joined with dbtab or not.
Related
I've recently taken over support of a system which uses Advantage Database Server as its back end. For some background, I have years of database experience but have never used ADS until now, so my question is purely about how to implement a standard pattern in this specific DBMS.
There's a stored procedure which has been previously developed which manages an ID column in this manner:
#ID = (SELECT ISNULL(MAX(ID), 0) FROM ExampleTable);
#ID = #ID + 1;
INSERT INTO Example_Table (ID, OtherStuff)
VALUES (#ID, 'Things');
--Do some other stuff.
UPDATE ExampleTable
SET AnotherColumn = 'FOO'
WHERE ID = #ID;
My problem is that I now need to run this stored procedure multiple times in parallel. As you can imagine, when I do this, the same ID value is getting grabbed multiple times.
What I need is a way to consistently create a unique value which I can be sure will be unique even if I run the stored procedure multiple times at the same moment. In SQL Server I could create an IDENTITY column called ID, and then do the following:
INSERT INTO ExampleTable (OtherStuff)
VALUES ('Things');
SET #ID = SCOPE_IDENTITY();
ADS has autoinc which seems similar, but I can't find anything conclusively telling me how to return the value of the newly created value in a way that I can be 100% sure will be correct under concurrent usage. The ADS Developer's Guide actually warns me against using autoinc, and the online help files offer functions which seem to retrieve the last generated autoinc ID (which isn't what I want - I want the one created by the previous statement, not the last one created across all sessions). The help files also list these functions with a caveat that they might not work correctly in situations involving concurrency.
How can I implement this in ADS? Should I use autoinc, some other built-in method that I'm unaware of, or do I genuinely need to do as the developer's guide suggests, and generate my unique identifiers before trying to insert into the table in the first place? If I should use autoinc, how can I obtain the value that has just been inserted into the table?
You use LastAutoInc(STATEMENT) with autoinc.
From the documentation (under Advantage SQL->Supported SQL Grammar->Supported Scalar Functions->Miscellaneous):
LASTAUTOINC(CONNECTION|STATEMENT)
Returns the last used autoinc value from an insert or append. Specifying CONNECTION will return the last used value for the entire connection. Specifying STATEMENT returns the last used value for only the current SQL statement. If no autoinc value has been updated yet, a NULL value is returned.
Note: Triggers that operate on tables with autoinc fields may affect the last autoinc value.
Note: SQL script triggers run on their own SQL statement. Therefore, calling LASTAUTOINC(STATEMENT) inside a SQL script trigger would return the lastautoinc value used by the trigger's SQL statement, not the original SQL statement which caused the trigger to fire. To obtain the last original SQL statement's lastautoinc value, use LASTAUTOINC(CONNECTION) instead.
Example: SELECT LASTAUTOINC(STATEMENT) FROM System.Iota
Another option is to use GUIDs.
(I wasn't sure but you may have already been alluding to this when you say "or do I genuinely need to do as the developer's guide suggests, and generate my unique identifiers before trying to insert into the table in the first place." - apologies if so, but still this info might be useful for others :) )
The use of GUIDs as a surrogate key allows either the application or the database to create a unique identifier, with a guarantee of no clashes.
Advantage 12 has built-in support for a GUID datatype:
GUID and 64-bit Integer Field Types
Advantage server and clients now support GUID and Long Integer (64-bit) data types in all table formats. The 64-bit integer type can be used to store integer values between -9,223,372,036,854,775,807 and 9,223,372,036,854,775,807 with no loss of precision. The GUID (Global Unique Identifier) field type is a 16-byte data structure. A new scalar function NewID() is available in the expression engine and SQL engine to generate new GUID. See ADT Field Types and Specifications and DBF Field Types and Specifications for more information.
http://scn.sap.com/docs/DOC-68484
For earlier versions, you could store the GUIDs as a char(36). (Think about your performance requirements here of course.) You will then need to do some conversion back and forth in your application layer between GUIDs and strings. If you're using some intermediary data access layer, e.g. NHibernate or Entity Framework, you should be able to at least localise the conversions to one place.
If some part of your logic is in a stored procedure, you should be able to use the newid() or newidstring() function, depending on the type of the backing column:
INSERT INTO Example_Table (newid(), OtherStuff)
I am scanning an SQLite database looking for all matches and using
OneFound:=False;
if tbl1.FieldByName('Name').AsString = 'jones' then
begin
OneFound:=True;
tbl1.Next;
end;
if OneFound then // Do something
or should I be using
if not(OneFound) then OneFound:=True;
Is it faster to just assign "True" to OneFound no matter how many times it is assigned or should I do the comparison and only change OneFuond the first time?
I know a better way would be to use FTS3, but for now I have to scan the database and the question is more on the approach to setting OneFound as many times as a match is encountered or using the compare-approach and setting it just once.
Thanks
Your question is, which is faster:
if not(OneFound) then OneFound:=True;
or
OneFound := True;
The answer is probably that the second is faster. Conditional statements involve branches which risks branch mis-prediction.
However, that line of code is trivial compared to what is around it. Running across a database one row at a time is going to be outrageously expensive. I bet that you will not be able to measure the difference between the two options because the handling of that little Boolean is simply swamped by the rest of the code. In which case choose the more readable and simpler version.
But if you care about the performance of this code you should be asking the database to do the work, as you yourself state. Write a query to perform the work.
It would be better to change your SQL statement so that the work is done in the database. If you want to know whether there is a tuple which contains the value 'jones' in the field 'name', then a quicker query would be
with tquery.create (nil) do
begin
sql.add ('select name from tbl1 where name = :p1 limit 1');
sql.params[0].asstring:= 'jones';
open;
onefound:= not isempty;
close;
free
end;
Your syntax may vary regarding the 'limit' clause but the idea is to return only one tuple from the database which matches the 'where' statement - it doesn't matter which one.
I used a parameter to avoid problems delimiting the value.
1. Search one field
If you want to search one particular field content, using an INDEX and a SELECT will be the fastest.
SELECT * FROM MYTABLE WHERE NAME='Jones';
Do not forget to create an INDEX on the column, first!
2. Fast reading
But if you want to search within a field, or within several fields, you may have to read and check the whole content. In this case, what will be slow will be calling FieldByName() for each data row: you should better use a local TField variable.
Or forget about TDataSet, and switch to direct access to SQLite3. In fact, using DB.pas and TDataSet requires a lot of data marshalling, so is slower than a direct access.
See e.g. DiSQLite3 or our DB classes, which are very fast, but a bit of higher level. Or you can use our ORM on top of those classes. Our classes are able to read more than 500,000 rows per second from a SQLite3 database, including JSON marshalling into objects fields.
3. FTS3/FTS4
But, as you guessed, the fastest would be indeed to use the FTS3/FTS4 feature of SQlite3.
You can think of FTS4/FTS4 as a "meta-index" or a "full-text index" on supplied blob of text. Just like google is able to find a word in millions of web pages: it does not use a regular database, but full-text indexing.
In short, you create a virtual FTS3/FTS4 table in your database, then you insert in this table the whole text of your main records in the FTS TEXT field, forcing the ID field to be the one of the original data row.
Then, you will query for some words on your FTS3/FTS4 table, which will give you the matching IDs, much faster than a regular scan.
Note that our ORM has dedicated TSQLRecordFTS3 / TSQLRecordFTS4 kind of classes for direct FTS process.
We are building ASP.NET MVC3 web applications using Visual Studio, SQL Server 2008 R2 & EF Code First 4.1.
Quite often we have smaller, what we call, "lookup" tables. For example a "Status" table contain an "Id" and a "Name". As the application grows these tables become quite frequent and I would like to know the best way to "group" these lesser important tables away from the crux of the application.
It has been suggest to me to add a prefix like "LkStatus" to help me but what about moving all the lookup tables out of dbo and into there own schema?
Can anyone see any drawbacks in this method?
Thanks Paul
No drawbacks with this method. I'm a fan of schemas personally. I'd use Lookup though
To change your table schema, you have two ways:
ALTER SCHEMA Lookup TRANSFER dbo.SomeTable
or
ALTER AUTHORIZATION ON dbo.SomeTable TO Lookup
This is going to be down to preference. There really isn't a "gotcha" either way. I prefer a table prefix but wouldn't be bothered either way. We use LU_*. As long as either option is enforced that maintenance down the line will be easy.
Since the tables are small, what about grouping them together into a single table? Instead of using the table name as a pseudo-key, use a real key. For example, you could have a table called Lookup, with an Id, Type, Name and Value, where Type = 'Status' for your status values. Seting the clustered index to (Type, Name) would physically group all rows of the same type together, which would make it fast to read them all as a group, if needed.
If your Names can have different data types, add an extra column for each required type: one for integers, one for strings, one for floats, etc. You can do something similar using an XML column; the T-SQL takes just a little more effort.
I'm creating a page where I want to make a history page. So I was wondering if there is any way to fetch all rows from multiple tables and then sort by their time? Every table has a field called "created_at".
So is there any way to fetch from all tables and sort without having Rails sorting them form me?
You may get a better answer, but I would presume you would need to
Create a History table with a Created date column, an autogenerated Id column, and any other contents you would like to expose [eg Name, Description]
Modify all tables that generate a "history" item to consume this new table via Foreign Key relationship on History.Id
"Mashing up" tables [ie merging different result sets into a single result set] is a very difficult problem, but you would effectively be doing the above anyway - just in the application layer, so why not do it correctly and more efficiently in the data layer.
Hope this helps :)
You would need to perform the sql like:
Select * from table order by created_at incr
: Store this into an array. Do this for each of the data sources, and then perform a merge sort on all the arrays in Ruby. Of course this will work well for small data sets, but once you get a data set that is large (ie: greater than will fit into memory) then you will have to use a different collect/merge algorithm.
So I guess the answer is that you do need to perform some sort of Ruby, unless you resort to the Union method described in another answer.
Depending on whether these databases are all on the same machine or not:
On same machine: Use OrderBy and UNION statements in your sql to return your result set
On different machines: You'll want to test this for performance, but you could use Linked Servers and UNION, ORDER BY. Alternatively, you could have ruby get the results from each db, and then combine them and sort
EDIT: From your last comment about different tables and not DB's; use something like this:
SELECT Created FROM table1
UNION
SELECT Created FROM table2
ORDER BY created
Let's say I have 'myStoredProcedure' that takes in an Id as a parameter, and returns a table of information.
Is it possible to write a SQL statement similar to this?
SELECT
MyColumn
FROM
Table-ify('myStoredProcedure ' + #MyId) AS [MyTable]
I get the feeling that it's not, but it would be very beneficial in a scenario I have with legacy code & linked server tables
Thanks!
You can use a table value function in this way.
Here is a few tricks...
No it is not - at least not in any official or documented way - unless you change your stored procedure to a TVF.
But however there are ways (read) hacks to do it. All of them basically involved a linked server and using OpenQuery - for example seehere. Do however note that it is quite fragile as you need to hardcode the name of the server - so it can be problematic if you have multiple sql server instances with different name.
Here is a pretty good summary of the ways of sharing data between stored procedures http://www.sommarskog.se/share_data.html.
Basically it depends what you want to do. The most common ways are creating the temporary table prior to calling the stored procedure and having it fill it, or having one permanent table that the stored procedure dumps the data into which also contains the process id.
Table Valued functions have been mentioned, but there are a number of restrictions when you create a function as opposed to a stored procedure, so they may or may not be right for you. The link provides a good guide to what is available.
SQL Server 2005 and SQL Server 2008 change the options a bit. SQL Server 2005+ make working with XML much easier. So XML can be passed as an output variable and pretty easily "shredded" into a table using the XML functions nodes and value. I believe SQL 2008 allows table variables to be passed into stored procedures (although read only). Since you cited SQL 2000 the 2005+ enhancements don't apply to you, but I mentioned them for completeness.
Most likely you'll go with a table valued function, or creating the temporary table prior to calling the stored procedure and then having it populate that.
While working on the project, I used the following to insert the results of xp_readerrorlog (afaik, returns a table) into a temporary table created ahead of time.
INSERT INTO [tempdb].[dbo].[ErrorLogsTMP]
EXEC master.dbo.xp_readerrorlog
From the temporary table, select the columns you want.