Someone might understand this ABL statement...
FOR EACH <table> BREAK BY <field as breakgroup>:
....
IF FIRST-OF(<breakgroup>) THEN DO:
....
END.
....
IF LAST-OF(<breakgroup>) THEN DO:
....
END.
....
END.
Above code basically loops through individual records in a table and then runs some codes (....) during specific parts of the execution. Interesting to note is the FIRST-OF and LAST-OF statement where the <breakgroup> value changes and is useful for data aggregation.
Does someone know how to implement/replicate the same logic using SQL perhaps with some VBA as well?
Firstly, it is worth noting that SQL, contrary to the OpenEdge ABL, is not a 4GL. It's not supposed to 'execute code' per se, so you will have to use another programming language to execute SQL statements against a Database, retrieve the selected records and then manipulate them with whatever language you might be using.
To get the equivalent of an ABL FIRST-OF or LAST-OF statement, the SQL FIRST() and LAST() function will come in handy (First() and Last() explain those.)
Basically, that means that you will have to do at least three different SQL Select statements I think, on to select all the table entries you want, one to select the first one of a given 'group' (a group, after all, is just a subselection of entries that fulfill certain criteria) and one to select the last one of such a group. For Example
SELECT LAST(CustomerType) WHERE CustomerType = "ValuedCustomer"
Now, if you want to use VBA as your programming language, you can execute SQL statements for instance with a command like this:
DoCmd.RunSQL "SELECT LAST(CustomerType) WHERE CustomerType = 'ValuedCustomer'"
Here is the MSDN entry for that command!
Hope that helped!
Related
I'm setting up an AR query that is basically meant to find an average of a few values that span three different tables. I'm getting hung up on how to embed the result of a particular Count query inside of the Active Record select statement.
Just by itself, this query returns "3":
Order.where(user_id: 319).count => 3
My question is, can I embed this into a select statement as a SQL alias similar to below:
Table.xxxxxx.select("Order.where(user_id: 319).count AS count,user_id, SUM(quantity*current_price) AS revenue").xxxxx
It seems to be throwing an error and generally not recognizing what I'm trying to do when I declare that first count alias. Any ideas on the syntax?
Well, after examining a bit, I cleared my mind into the ActiveRecord select() syntax.
It's a method that can take a variable length of parameters. So, your failing :
Table.xxxxxx.select("Order.where(user_id: 319).count AS count,user_id, SUM(quantity*current_price) AS revenue").xxxxx
After replacing proper SQL for your misplaced ActiveRecord statement, should be more of like this [be careful, you can't use as count in most cases, count is reserved]:
Table.xxxxx.select("(SELECT count(id) from orders where user_id=319) as usercount", "user_id","SUM(quantity*current_price) AS revenue").xxxx
But I guess you should need more a per-user_id-table.
So, I'd skip Models and go to direct SQL, always being careful to avoid injections:
ActiveRecord::Base.connection.execute('SELECT COUNT(orders.id) as usercount, users.id from users, orders where users.id=orders.user_id group by users.id')
This is simplified of course, you can apply the rest of the data (which I currently do not know) accordingly. The above simplified, not full solution, could be written also as:
Order.joins(:user).select("count(orders.id) as usercount, users.id").group(:user_id)
I am trying to determine if I should use a CASE Statement or an IF THEN statement to get my results.
I want a SQL statement to run when a certain condition exists, but am not certain on how to check for the condition. Here is what I am working on
IF EXISTS(SELECT source FROM map WHERE rev_num =(SELECT MAX(rev_num) from MAP <-- at this point it would return either an A or B -->
What ever the answer is i then need to run a set of SQL's. So for A it would do this set of statements and for B it would do another.
CASE is used within a SQL statement. IF/THEN can be used to choose which query to execute.
Based on your somewhat vague example it seems like you want to execute different queries based on some condition. In that case, an IF/THEN seems more appropriate.
If, however, the majority of each query is identical and you're just changing part of the query then you may be able to use CASE to reduce the amount of duplicate code.
My colleague wrote this SQL statement and I had a hard time understanding it. What exactly is the purpose of using a colon in the where clause?
WHERE MGM_YYMM like :AS_YYMM
Full Query:
SELECT A.MGM_YYMM,
A.MGM_DATE,
A.MGM_GB,
A.INDATE,
A.SUDATE,
A.EMPNUM,
FROM SE_MAGAM A(NOLOCK)
WHERE MGM_YYMM like :AS_YYMM
ORDER BY MGM_YYMM DESC
It is a bind variable.
The program (or whatever else is issuing the query) will assign a value to :AS_YYMM, in this case the pattern to match against the MGM_YYMM column.
These kind of parameterized queries are useful because they can be prepared/parsed/compiled/analyzed once and then be run multiple times for varying inputs with reduced overhead (compared to a new query each time). Also helps against SQL injection (compared to building a dynamic SQL statement from user input).
I suppose I have always naively assumed that scalar functions in the select part of a SQL query will only get applied to the rows that meet all the criteria of the where clause.
Today I was debugging some code from a vendor and had that assumption challenged. The only reason I can think of for this code failing is that the Substring() function is getting called on data that should have been filtered out by the WHERE clause. But it appears that the substring call is being applied before the filtering happens, the query is failing.
Here is an example of what I mean. Let's say we have two tables, each with 2 columns and having 2 rows and 1 row respectively. The first column in each is just an id. NAME is just a string, and NAME_LENGTH tells us how many characters in the name with the same ID. Note that only names with more than one character have a corresponding row in the LONG_NAMES table.
NAMES: ID, NAME
1, "Peter"
2, "X"
LONG_NAMES: ID, NAME_LENGTH
1, 5
If I want a query to print each name with the last 3 letters cut off, I might first try something like this (assuming SQL Server syntax for now):
SELECT substring(NAME,1,len(NAME)-3)
FROM NAMES;
I would soon find out that this would give me an error, because when it reaches "X" it will try using a negative number for in the substring call, and it will fail.
The way my vendor decided to solve this was by filtering out rows where the strings were too short for the len - 3 query to work. He did it by joining to another table:
SELECT substring(NAMES.NAME,1,len(NAMES.NAME)-3)
FROM NAMES
INNER JOIN LONG_NAMES
ON NAMES.ID = LONG_NAMES.ID;
At first glance, this query looks like it might work. The join condition will eliminate any rows that have NAME fields short enough for the substring call to fail.
However, from what I can observe, SQL Server will sometimes try to calculate the the substring expression for everything in the table, and then apply the join to filter out rows. Is this supposed to happen this way? Is there a documented order of operations where I can find out when certain things will happen? Is it specific to a particular Database engine or part of the SQL standard? If I decided to include some predicate on my NAMES table to filter out short names, (like len(NAME) > 3), could SQL Server also choose to apply that after trying to apply the substring? If so then it seems the only safe way to do a substring would be to wrap it in a "case when" construct in the select?
Martin gave this link that pretty much explains what is going on - the query optimizer has free rein to reorder things however it likes. I am including this as an answer so I can accept something. Martin, if you create an answer with your link in it i will gladly accept that instead of this one.
I do want to leave my question here because I think it is a tricky one to search for, and my particular phrasing of the issue may be easier for someone else to find in the future.
TSQL divide by zero encountered despite no columns containing 0
EDIT: As more responses have come in, I am again confused. It does not seem clear yet when exactly the optimizer is allowed to evaluate things in the select clause. I guess I'll have to go find the SQL standard myself and see if i can make sense of it.
Joe Celko, who helped write early SQL standards, has posted something similar to this several times in various USENET newsfroups. (I'm skipping over the clauses that don't apply to your SELECT statement.) He usually said something like "This is how statements are supposed to act like they work". In other words, SQL implementations should behave exactly as if they did these steps, without actually being required to do each of these steps.
Build a working table from all of
the table constructors in the FROM
clause.
Remove from the working table those
rows that do not satisfy the WHERE
clause.
Construct the expressions in the
SELECT clause against the working table.
So, following this, no SQL dbms should act like it evaluates functions in the SELECT clause before it acts like it applies the WHERE clause.
In a recent posting, Joe expands the steps to include CTEs.
CJ Date and Hugh Darwen say essentially the same thing in chapter 11 ("Table Expressions") of their book A Guide to the SQL Standard. They also note that this chapter corresponds to the "Query Specification" section (sections?) in the SQL standards.
You are thinking about something called query execution plan. It's based on query optimization rules, indexes, temporaty buffers and execution time statistics. If you are using SQL Managment Studio you have toolbox over your query editor where you can look at estimated execution plan, it shows how your query will change to gain some speed. So if just used your Name table and it is in buffer, engine might first try to subquery your data, and then join it with other table.
My company has just started using LINQ and I still am having a little trouble with the abstractness (if thats a word) of the LINQ command and the SQL, my question is
Dim query = (From o In data.Addresses _
Select o.Name).Count
In the above in my mind, the SQL is returning all rows and the does a count on the number rows in the IQueryable result, so I would be better with
Dim lstring = Aggregate o In data.Addresses _
Into Count()
Or am I over thinking the way LINQ works ? Using VB Express at home so I can't see the actual SQL that is being sent to the database (I think) as I don't have access to the SQL profiler
As mentioned, these are functionally equivalent, one just uses query syntax.
As mentioned in my comment, if you evaluate the following as a VB Statement(s) in LINQPad:
Dim lstring = Aggregate o In Test _
Into Count()
You get this in the generated SQL output window:
SELECT COUNT(*) AS [value]
FROM [Test] AS [t0]
Which is the same as the following VB LINQ expression as evaluated:
(From o In Test_
Select o.Symbol).Count
You get the exact same result.
I'm not familiar with Visual Basic, but based on
http://msdn.microsoft.com/en-us/library/bb546138.aspx
Those two approaches are the same. One uses method syntax and the other uses query syntax.
You can find out for sure by using SQL Profiler as the queries run.
PS - The "point" of LINQ is you can easily do query operations without leaving code/VB-land.
An important thing here, is that the code you give will work with a wide variety of data sources. It will hopefully do so in a very efficient way, though that can't be fully guaranteed. It certainly will be done in an efficient way with a SQL source (being converted into a SELECT COUNT(*) SQL query. It will be done efficiently if the source was an in-memory collection (it gets converted to calling the Count property). It isn't done very efficiently if the source is an enumerable that is not a collection (in this case it does read everything and count as it goes), but in that case there really isn't a more efficient way of doing this.
In each case it has done the same conceptual operation, in the most efficient manner possible, without you having to worry about the details. No big deal with counting, but a bigger deal in more complex cases.
To a certain extent, you are right when you say "in my mind, the SQL is returning all rows and the does a count on the number rows". Conceptually that is what is happening in that query, but the implementation may differ. Compare with how the real query in SQL may not match the literal interpretation of the SQL command, to allow the most efficient approach to be picked.
I think you are missing the point as Linq with SQL has late binding the search is done when you need it so when you say I need the count number then a Query is created.
Before that Linq for SQL creates Expression trees that will be "translated" in to SQL when you need it....
http://weblogs.asp.net/scottgu/archive/2007/05/19/using-linq-to-sql-part-1.aspx
http://msdn.microsoft.com/en-us/netframework/aa904594.aspx
How to debug see Scott
http://weblogs.asp.net/scottgu/archive/2007/07/31/linq-to-sql-debug-visualizer.aspx
(source: scottgu.com)