I have enherited an Access-database with some queries. It is an account-database with information on the different accounts. Specifically IBAN-numbers for each account.
One of the queries is this, where we compare IBAN-numbers from the database with IBAN-numbers from the imported Id-table:
SELECT CAMTaccounts.IBAN, CAMTaccounts.Comment
FROM CAMTaccounts LEFT JOIN Id ON CAMTaccounts.[IBAN] = Id.[IBAN]
WHERE (((Id.IBAN) Is Null));
I thought I understood the SQL-language to some degree coming from SQL Server, but this statement, I cannot understand.
To me, this is equivalent to writing:
Select CAMTaccounts.*
From CAMTaccounts Left Outer Join Id On CAMTaccounts.IBAN = Id.IBAN
Where Id.IBAN Is Null
and this join, to me, does not make any sense.
But clearly, I am not understanding this correctly.
I was hoping, that some of you could explain my flawed logic to me.
Thanks.
This query will return all rows from CAMTaccounts which have no matching Id rows. This is known as Anti-join query. Excessive parenthesis really make no sense, most probably generated with a tool.
Suppose that CAMTaccounts.IBAN have values
A
B
And Id.IBAN have values
B
C
Then this query WITHOUT WHERE clause outputs something like this
`CAMTaccounts.IBAN` | `Id.IBAN`
---------------------------------
A | NULL
B | B
You can easily see what will result from this, if add Where Id.IBAN Is Null clause.
So, this query will give you result from CAMTaccounts table, when IBAN column doesn't contains value from Id.IBAN column
Related
I'm trying to build a sub-query with a list in the where clause, I have tried several variations and I think the problem is with the way I'm structuring the WHERE IN. Help is grealy appreciated!!
SELECT a.ACCT_SK,
a.BTN,
a.PRODUCT_SET,
MAX(b.ORD_CREATD_DT)
FROM MM.MEC_ACCT_ATTR a, CDI_CRM.ORD_MSTR b
WHERE a.ACCT_SK=b.ACCT_SK AND a.BTN=b.BTN
(SELECT b.ACCT_SK, b.ORD_CREATD_DT
FROM CDI_CRM.ORD_MSTR b
WHERE b.ACCT_SK IN ('44347714',
'44023302',
'43604964'));
SELECT Failed. 3706: (-3706)Syntax error: expected something between '(' and the 'SELECT' keyword
The desired output is a table with Product set for 50 ACCT_SKs with the most recent order date matched on ACCT_SK and BTN.
Sample data and desired results would really help. Your query doesn't make much sense, but I suspect you want:
SELECT a.ACCT_SK, a.BTN, a.PRODUCT_SET,
MAX(o.ORD_CREATD_DT)
FROM MM.MEC_ACCT_ATTR a JOIN
CDI_CRM.ORD_MSTR o
ON a.ACCT_SK = o.ACCT_SK AND a.BTN = o.BTN
WHERE a.ACCT_SK IN ('44347714', '44023302', '43604964')
GROUP BY a.ACCT_SK, a.BTN, a.PRODUCT_SET;
This returns the columns you want for the three specified accounts.
Notes:
Always use proper, explicit, standard JOIN syntax. Never use commas in the FROM clause.
Your subquery simply makes no sense. It is not connected to anything else in the query.
You are using an aggregation function (MAX()) so your query is an aggregation query and needs a GROUP BY.
Use meaningful table aliases. a makes sense for an accounts table, but b does not make sense for an orders table.
I am working on 2 different assignments where I have to do null checks but I'm not sure if I have written the syntax correctly for that my instructor has not really discussed this but will be marking for it.
Below are the 2 questions and what I have written. Any help is appreciated.
Assignment 1 question: Create a list of the sales order numbers for orders not ordered online and not with a credit card. Note: 0 is false and 1 is true for bit fields. Below is the syntax i used, am i doing a null check here?
SELECT SalesOrderNumber
FROM Sales.SalesOrder_json
WHERE OnlineOrderFlag = 0 AND CreditCardID IS NULL
Assignment 2 question: list the vendors that have no products. Below is the syntax I used, am I doing a null check here?
SELECT
pv.Name AS Vendors,
COUNT(PP.ProductID) AS 'Products'
FROM
Purchasing.Vendor AS PV
LEFT JOIN
Purchasing.ProductVendor AS PPV ON PV.BusinessEntityID = PPV.BusinessEntityID
LEFT JOIN
Production.Product AS PP ON PP.ProductID = PPV.ProductID
GROUP BY
PV.Name
HAVING
COUNT(PP.ProductID) = 0;
Welcome to Stack Overflow!
In the future, please post a summary or create table statements that represents the schema of the tables used in your queries so that we have enough information to provide more than speculative responses. Even though this is the Adventure Works DB, you should start your SO journey with good habits!
please try not to post direct Assignment questions online as you will easily get done for plagiarism by most academic assignment checkers, mainly because other students may see your post, and the support that you get from the community which could result in all of you handing in the same result.
Have you run your queries? Do you think the results are correct?
If the results from your queries are correct, then the only issue is "have you done any null checks"? One could say that if your results have returned the correct results then you must have satisfied the criteria, otherwise the question wasn't formulated very well.
Null checks can be summarised into 3 patterns:
You directly compare against null using IS NULL or IS NOT NULL in your query
Use of JOIN syntax to deal with data that may have nulls.
INNER JOIN will limit the results to only records that match in both tables. Use this if you need to omit records that have a null in the foreign key field.
Non INNER joins, like LEFT JOIN. This will return results from the left table, even if there are no matching records in the joined or right table.
This is a good discussion on all supported joins: LEFT JOIN vs. LEFT OUTER JOIN in SQL Server
Use of Aggregation functions, aggregates will generally omit null values, COUNT will return 0 if all values are NULL, where as other aggregates such as SUM, MIN, MAX, AVG will return NULL if all values are NULL
Question 1
Clearly you have implemented a NULL check because you have evaluated criteria directly on the nullable column.
It looks like your answer to Question 1 is pretty good.
Question 2
While your query looks like it would return the vendors with no products, it is also returning a count of zero.
You do not need to output a column so that you can use it in a filter criteria, so remove COUNT(PP.ProductID) AS 'Products' unless you have been otherwise instructed to use it.
Is this a NULL check... That up to the interpretation, I think in this case the answer is yes. By using LEFT JOIN (or OUTER joins) you have created a result set that will have the field PP.ProductID with a value of NULL If there are no products.
Using Count in the filter criteria over that null column and recognising that a Count with a zero result means that the ProductID column was in fact null means you have evaluated a null check.
There are other ways to query for the same results, such as using NOT EXISTS. NOT EXISTS would NOT be a direct null check, because NULLABILITY was not evaluated directly.
I need to flatten out 2 rows in a vertical table (and then join to a third table) I generally do this by making a derived table for each field I need. There's only two fields, I figure this isn't that unreasonable.
But I know that the rows I want back in the derived table, are the subset that's in my join with my third table.
So I'm trying to figure out the best derived tables to make so that the query runs most efficiently.
I figure the more restrictive I make the derived table's where clause, the smaller the derived table will be, the better response I'll get.
Really what I want is to correlate the where clause of the derived table with the join with the 3rd table, but you can't do that in sql, which is too bad. But I'm no sql master, maybe there's some trick I don't know about.
The other option is just to make the derived table(s) with no where clause and it just ends up joining the entire table twice (once for each field), and when I do my join against them the join filters every thing out.
So really what I'm asking I guess is what's the best way to make a derived table where I know pretty much specifically what rows I want, but sql won't let me get at them.
An example:
table1
------
id tag value
-- ----- -----
1 first john
1 last smith
2 first sally
2 last smithers
table2
------
id occupation
-- ----------
1 carpenter
2 homemaker
select table2.occupation, firsttable.first, lasttable.last from
table2, (select value as first from table1 where tag = 'first') firsttable,
(select value as last from table1 where tag = 'last') lasttable
where table2.id = firsttable.id and table2.id = lasttable.id
What I want to do is make the firsttable where clause where tag='first' and id = table2.id
DERIVED tables are not to store the intermediate results as you expect. These are just a way to make code simpler. Using derived table doesnt mean that the derived table expression will be executed first and output of that will be used to join with remaining tables.Optimizer will automaticaly faltten derived tables in most of the cases.
However,There are cases where the optimizer might want to store the results of the subquery and thus materilize instead of flattening.It usually happens when you have some kind of aggregate functions or like that.But in your case the query is too simple and thus optimizer will flatten query
Also,storing derived table expression wont make your query fast it will in turn could make it worse.Your real problem is too much normalization.Fix that query will be just a join of two tables.
Why you have this kind of normalization?Why you are storing col values as rows.Try to denormalize table1 so that it has two columns first and last.That will be best solution for this.
Also, do you have proper indexes on id and tag column? if yes then a merge join is quite good for your query.
Please provide index details on these tables and the plan generated by your query.
Your query will be used like an inner join query.
select table2.occupation, first.valkue as first, last.value as last
from
table2
inner join table1 first
on first.tag = 'first'
and first.id =table2.id
inner join table1 last
on last.tag = 'last'
and table2.id = last.id
I think what you're asking for is a COMMON TABLE EXPRESSION. If your platform doesn't implement them, then a temporary table may be the best alternative.
I'm a little confused. Your query looks okay . . . although it looks better with proper join syntax.
select table2.occupation, firsttable.first, lasttable.last
from table2 join
(select value as first from table1 where tag = 'first') firsttable
on table2.id = firsttable.id join
(select value as last from table1 where tag = 'last') lasttable
on table2.id = lasttable.id
This query does what you are asking it to do. SQL is a declarative language, not a procedural language. This means that you describe the result set and rely on the database SQL compiler to turn it into the right set of commands. (That said, sometimes how a query is structured does make it easier or harder for some engines to produce efficient query plans.)
Just need some quick clarification
I have 2 Queries in my Access Database that should return Inverse results:
SELECT Equipment.title
FROM Equipment
WHERE (((Equipment.[EquipmentID]) Not In (
select EquipmentID
from DownPeriod
where UpDate is null
)));
The 2nd just excludes the Not before the In.
My Confusion comes from the fact that the query posted above does not return any results if an EquipmentID field has at least 1 null value in the DownPeriod table.
It works fine if the fields are filled, and the inverse query list always works. This makes me think there's an issue with the null value.
Now this field should never be null but I wanted to know if I could still get this to work in the unlikely event a null did occur.
Thank you in advanced!
Try joins:
SELECT Equipment.title FROM Equipment INNER JOIN DownPeriod
ON Equipment.EquipmentID = DownPeriod.EquipmentID
WHERE DownPeriod.UpDate is null
and
SELECT Equipment.title FROM Equipment INNER JOIN DownPeriod
ON Equipment.EquipmentID = DownPeriod.EquipmentID
WHERE DownPeriod.UpDate is not null
See if a change in syntax fixes your issue.
Not only should this work, but I believe it is a faster practise than using the IN() NOT IN() methods (might be wrong on that, but it looks nicer to read). It also adds the ability to quickly change the "is not null" criteria just the same as IN->NOT IN
I agree with StuckAtWork's approach. However, if you still want to understand why your original approach didn't produce the results you want, I think I can help you.
There may be an issue with empty strings which could complicate the situation. But regardless of whether or not empty strings are involved you have something more fundamental to consider.
Here is my version of the Equipment table.
EquipmentID title
1 one
2 two
3 three
And here is my version of the DownPeriod table.
ID EquipmentID text_field
1 1 one
2 2 two
3 Null
4 3 three
I didn't include your UpDate field in my DownPeriod table. It's irrelevant to your problem.
I pasted your SQL into a new Access query, discarded the WHERE clause from the subquery, and got exactly the same result as this query --- no rows returned:
SELECT e.title
FROM Equipment AS e
WHERE
e.EquipmentID Not In (
SELECT EquipmentID
FROM DownPeriod
);
So consider this situation from the db engine's perspective. Using my version of the Downloads table, it has a set of values (1, 2, Null, and 3) from the subquery. You're asking it to show you the rows from Equipment where EquipmentID is NOT IN that list of values. The db engine will only give you the rows for which that condition is True.
Null is the problem. For each EquipmentID, when it considers whether that value is not present in the subquery set, it doesn't know. That Null is an unknown value ... and the unknown value might be the same as the current EquipmentID it's considering ... or might be something else. But since the db engine doesn't know the real value, it can't evaluate the condition as True, so will not include that row in the result set. The same thing happens for every row in Equipment table ... therefore your query's result set is empty (no rows).
You could get your desired results by excluding Null values from the subquery result set with a WHERE clause like the one below. But I think StuckAtWork's suggestion is a better way to go.
SELECT e.title
FROM Equipment AS e
WHERE
e.EquipmentID Not In (
SELECT EquipmentID
FROM DownPeriod
WHERE EquipmentID Is Not Null
);
There must be something I'm missing here. I have this nice, pretty Oracle SQL statement in Toad that gives me back a list of all active personnel with the IDs that I want:
SELECT PERSONNEL.PERSON_ID,
PERSONNEL.NAME_LAST_KEY,
PERSONNEL.NAME_FIRST_KEY,
PA_EID.ALIAS EID,
PA_IDTWO.ALIAS IDTWO,
PA_LIC.ALIAS LICENSENO
FROM PERSONNEL
LEFT JOIN PERSONNEL_ALIAS PA_EID
ON PERSONNEL.PERSON_ID = PA_EID.PERSON_ID
AND PA_EID.PERSONNEL_ALIAS_TYPE_CD = 1086
AND PA_EID.ALIAS_POOL_CD = 3796547
AND PERSONNEL.ACTIVE_IND = 1
LEFT JOIN PERSONNEL_ALIAS PA_IDTWO
ON PERSONNEL.PERSON_ID = PA_IDTWO.PERSON_ID
AND PA_IDTWO.PERSONNEL_ALIAS_TYPE_CD = 3839085
AND PA_IDTWO.ACTIVE_IND = 1
LEFT JOIN PERSONNEL_ALIAS PA_LIC
ON PERSONNEL.PERSON_ID = PA_LIC.PERSON_ID
AND PA_LIC.PERSONNEL_ALIAS_TYPE_CD = 1087
AND PA_LIC.ALIAS_POOL_CD = 683988
AND PA_LIC.ACTIVE_IND = 1
WHERE PERSONNEL.ACTIVE_IND = 1 AND PERSONNEL.PHYSICIAN_IND = 1;
This works very nicely. Where I run into problems is when I put it into Access. I know, I know, Access Sucks. Sometimes one needs to use it, especially if one has multiple database types that they just want to store a few queries in, and especially if one's boss only knows Access. Anyway, I was having trouble with the ANDs inside the FROM, so I moved those to the WHERE, but for some odd reason, Access isn't doing the LEFT JOINs, returning only those personnel with EID, IDTWO, and LICENSENO's. Not everybody has all three of these.
Best shot in Access so far is:
SELECT PERSONNEL.PERSON_ID,
PERSONNEL.NAME_LAST_KEY,
PERSONNEL.NAME_FIRST_KEY,
PA_EID.ALIAS AS EID,
PA_IDTWO.ALIAS AS ID2,
PA_LIC.ALIAS AS LICENSENO
FROM ((PERSONNEL
LEFT JOIN PERSONNEL_ALIAS AS PA_EID ON PERSONNEL.PERSON_ID=PA_EID.PERSON_ID)
LEFT JOIN PERSONNEL_ALIAS AS PA_IDTWO ON PERSONNEL.PERSON_ID=PA_IDTWO.PERSON_ID)
LEFT JOIN PERSONNEL_ALIAS AS PA_LIC ON PERSONNEL.PERSON_ID=PA_LIC.PERSON_ID
WHERE (((PERSONNEL.ACTIVE_IND)=1)
AND ((PERSONNEL.PHYSICIAN_IND)=1)
AND ((PA_EID.PRSNL_ALIAS_TYPE_CD)=1086)
AND ((PA_EID.ALIAS_POOL_CD)=3796547)
AND ((PA_IDTWO.PRSNL_ALIAS_TYPE_CD)=3839085)
AND ((PA_IDTWO.ACTIVE_IND)=1)
AND ((PA_LIC.PRSNL_ALIAS_TYPE_CD)=1087)
AND ((PA_LIC.ALIAS_POOL_CD)=683988)
AND ((PA_LIC.ACTIVE_IND)=1));
I think that part of the problem could be that I'm using the same alias (lookup) table for all three joins. Maybe there's a more efficient way of doing this? Still new to SQL land, so any tips as far as that goes would be great. I feel like these should be equivalent, but the Toad query gives me back many many tens of thousands of imperfect rows, and Access gives me fewer than 500. I need to find everybody so that nobody is left out. It's almost as if the LEFT JOINs aren't working at all in Access.
To understand what you are doing, let's look at simplified version of your query:
SELECT PERSONNEL.PERSON_ID,
PA_EID.ALIAS AS EID
FROM PERSONNEL
LEFT JOIN PERSONNEL_ALIAS AS PA_EID ON PERSONNEL.PERSON_ID=PA_EID.PERSON_ID
WHERE PERSONNEL.ACTIVE_IND=1
AND PERSONNEL.PHYSICIAN_IND=1
AND PA_EID.PRSNL_ALIAS_TYPE_CD=1086
AND PA_EID.ALIAS_POOL_CD=3796547
If the LEFT JOIN finds match, your row might look like this:
Person_ID EID
12345 JDB
If it doesn't find a match, (disregard the WHERE clause for a second), it could look like:
Person_ID EID
12345 NULL
When you add the WHERE clauses above, you are telling it to only find records in the PERSONNEL_ALIAS table that meet the condition, but if no records are found, then the values are considered NULL, so they will never satisfy the WHERE condition and no records will come back...
As Joe Stefanelli said in his comment, adding a WHERE clause to a LEFT JOIN'ed table make it act as an INNER JOIN instead...
Further to #Sparky's answer, to get the equivalent of what you're doing in Oracle, you need to filter rows from the tables on the "outer" side of the joins before you join them. One way to do this might be:
For each table on the "outer" side of a join that you need to filter rows from (that is, the three instances of PERSONNEL_ALIAS), create a query that filters the rows you want. For example, the first query (say, named PA_EID) might look something like this:SELECT PERSONNEL_ALIAS.* FROM PERSONNEL_ALIAS WHERE PERSONNEL_ALIAS.PERSONNEL_ALIAS_TYPE_CD = 1086 AND PERSONNEL_ALIAS.ALIAS_POOL_CD = 3796547
In your "best shot in Access so far" query in the original post: a) replace each instance of PERSONNEL_ALIAS with the corresponding query created in Step 1, and, b) remove the corresponding conditions (on PA_EID, PA_IDTWO, and PA_LIC) from the WHERE clause.