I was going through the code, I found one query where "?"mentioned.
query is
select distinct from A inner join B ON A.c =B.a and A.x=B.f and A.x=?
here what is the meaning of A.x=? , not able to understand.
is it something with runtime value, if yes then how ? will take runtime parameter.
I'm guessing this is from SSIS.
When you use a parameterised query over anything but ADO.NET, the parameters are defined with question marks. The first question mark is parameter 1, the second is parameter 2 etc. The problem with this is that each parameter can only be used once.
With ADO.NET you can specify parameter names which (in my view) is much better.
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)
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).
Is there a way to reference field name with a variable value in Firebird ?
for example I want to make SQL like this:
Insert into tab1 (1, f1, f2, f3)
select 1, tab2.f+var_loop, tab2.f+var_loop, tab2.f+var_loop
from tab2
where .....
where "f" is the first initial of the field name and "var_loop" is a loop variable
Thanks
No, this is not supported. I quess you can achieve something like this using EXECUTE BLOCK, but then you could build the right SQL statement on the client side right away too, would be much easier...
To do this, you must write your application code to build the SQL prior to preparing the query.
In SQL, the name and number of columns must be fixed at the time you prepare the query. Column names can't be based on expressions that aren't evaluated until runtime. This is true in standard SQL and in every brand of RDBMS as far as I know.
Also you can't use parentheses around a list of column in a SELECT clause as you're doing.
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.
This question already has answers here:
Subquery using Exists 1 or Exists *
(6 answers)
Closed 7 years ago.
I've seen some people use EXISTS (SELECT 1 FROM ...) rather than EXISTS (SELECT id FROM ...) as an optimization--rather than looking up and returning a value, SQL Server can simply return the literal it was given.
Is SELECT(1) always faster? Would Selecting a value from the table require work that Selecting a literal would avoid?
In SQL Server, it does not make a difference whether you use SELECT 1 or SELECT * within EXISTS. You are not actually returning the contents of the rows, but that rather the set determined by the WHERE clause is not-empty. Try running the query side-by-side with SET STATISTICS IO ON and you can prove that the approaches are equivalent. Personally I prefer SELECT * within EXISTS.
For google's sake, I'll update this question with the same answer as this one (Subquery using Exists 1 or Exists *) since (currently) an incorrect answer is marked as accepted. Note the SQL standard actually says that EXISTS via * is identical to a constant.
No. This has been covered a bazillion times. SQL Server is smart and knows it is being used for an EXISTS, and returns NO DATA to the system.
Quoth Microsoft:
http://technet.microsoft.com/en-us/library/ms189259.aspx?ppud=4
The select list of a subquery
introduced by EXISTS almost always
consists of an asterisk (*). There is
no reason to list column names because
you are just testing whether rows that
meet the conditions specified in the
subquery exist.
Also, don't believe me? Try running the following:
SELECT whatever
FROM yourtable
WHERE EXISTS( SELECT 1/0
FROM someothertable
WHERE a_valid_clause )
If it was actually doing something with the SELECT list, it would throw a div by zero error. It doesn't.
EDIT: Note, the SQL Standard actually talks about this.
ANSI SQL 1992 Standard, pg 191 http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt
3) Case:
a) If the <select list> "*" is simply contained in a <subquery> that is immediately contained in an <exists predicate>, then the <select list> is equivalent to a <value expression> that is an arbitrary <literal>.
When you use SELECT 1, you clearly show (to whoever is reading your code later) that you are testing whether the record exists. Even if there is no performance gain (which is to be discussed), there is gain in code readability and maintainability.
Yes, because when you select a literal it does not need to read from disk (or even from cache).
doesn't matter what you select in an exists clause. most people do select *, then sql server automatically picks the best index
As someone pointed out sql server ignores the column selection list in EXISTS so it doesn't matter. I personally tend to use "SELECT null ..." to indicate that the value is not used at all.
If you look at the execution plan for
select COUNT(1) from master..spt_values
and look at the stream aggregate you will see that it calculates
Scalar Operator(Count(*))
So the 1 actually gets converted to *
However I have read somewhere in the "Inside SQL Server" series of books that * might incur a very slight overhead for checking column permissions. Unfortunately the book didn't go into any more detail than that as I recall.
Select 1 should be better to use in your example. Select * gets all the meta-data assoicated with the objects before runtime which adss overhead during the compliation of the query. Though you may not see differences when running both types of queries in your execution plan.