I really don't understand why my statement:
IF EXISTS (SELECT * FROM people WHERE ID = 168)
THEN SELECT * FROM people
END IF;
is returning this error:
Unknown statement type. (near "IF EXISTS" at position 0)
I'm using MariaDB 10.3. Any ideas?
ADDITIONAL INFO
This is of course a simplified example. What I wanna do is, concretely:
IF EXISTS (SELECT * FROM people WHERE ID = 168)
THEN UPDATE people SET calculated_value = complex_queries_and_calculations
WHERE ID = 168
.., so to update a field of a given record if that record contains a given data, and else do nothing.
To generate the data which would be used for the update, I need to query other tables for values and make some calculations. I want to avoid these queries + calculations, if there's actually nothing to update. And in this case, simply do nothing.
Hence, I guess that putting for example an EXIST clause inside a WHERE clause of the UPDATEstatement would end in many queries and calculations made in vain.
MySQL and MariaDB only allow IF statements in programming blocks -- stored functions, procedures, and triggers.
Instead, just use:
select p.*
from people p
where exists (select 1 from people p2 where p2.id = 168);
This returns all people if id 168 is in table.
Related
I've been provided the below schema for this problem and I'm trying to do two things:
Update the ACCOUNT table's average_eval row with the average of the evaluation row from the POST_EVAL table per account_id.
Update the ACCOUNT table with a count of the number of posts per account_id, with default value 0 if the account_id has no post_id associated to it.
Here's the kicker : I MUST use the UPDATE statement and I'm not allowed to use triggers for these specific problems.
I've tried WITH clauses and GROUP BY but haven't gotten anywhere. Using postresql's pgadmin for reference.
Any help setting up these queries?
The first question can be done using something like this:
update account a
set average_eval = t.avg_eval
from (
select account_id, avg(evaluation) as avg_eval
from post_eval
group by account_id
) t
where t.account_id = a.account_id
The second question needs a co-related sub-query as there is no way to express an outer join in an UPDATE statement like the above:
update account a
set num_posts = (select count(*)
from post p
where p.account_id = a.account_id);
The count() will return zero (0) if there are no posts for that account. If a join was used (as in the first statement), the rows would not be updated at all, as the "join" condition wouldn't match.
I have not tested either of those statements, so they can contain typos (or even logical errors).
Unrelated, but: I understand that this is some kind of assignment, so you have no choice. But as RiggsFolly has mentioned: in general you should avoid storing information in a relational database that can be derived from existing data. Both values can easily be calculated in a view and then will always be up-to-date.
I'm testing this UPDATE statement to update all 4%, 8%, and 9% parts in our database. I'm trying to get the QTY_MULTIPLE value to match the CASES per layer value.
So, the REGEXP_LIKE, will eventually read:
> Regexp_like ( sp.part_no, '^4|^8|^9' )
It doesn't right now because I'm testing three specific parts. I want to make sure the rest of the statement works the way that it's supposed to.
Here's what I'm testing with:
UPDATE SALES_PART_TAB sp
SET sp.qty_multiple = ( SELECT cases_per_layer
FROM HH_INV_PART_CHARS
WHERE sp.part_no = HH_INV_PART_CHARS.part_no AND
sp.contract = HH_INV_PART_CHARS.contract )
WHERE Regexp_like ( sp.part_no, '^410-0017|^816-0210|^921-0003' ) AND
EXISTS
( SELECT sp.contract,
sp.part_no,
sp.qty_multiple,
HH_INV_PART_CHARS.cases_per_layer
FROM SALES_PART sp
inner join HH_INV_PART_CHARS
ON sp.part_no = HH_INV_PART_CHARS.part_no AND
sp.contract = HH_INV_PART_CHARS.contract
WHERE sp.qty_multiple != HH_INV_PART_CHARS.cases_per_layer );
When I run this statement, it updates 16 rows.
However, I'm expecting it to update 15 rows. I reached this conclusion by running the following SELECT statement:
SELECT sp.contract,
sp.catalog_no,
sp.qty_multiple,
HH_INV_PART_CHARS.cases_per_layer
FROM SALES_PART sp
inner join HH_INV_PART_CHARS
ON sp.part_no = HH_INV_PART_CHARS.part_no AND
sp.contract = HH_INV_PART_CHARS.contract
WHERE sp.qty_multiple != HH_INV_PART_CHARS.cases_per_layer AND
Regexp_like ( sp.part_no, '^410-0017|^816-0210|^921-0003' )
I think the problem I'm having is the UPDATE statement is updating all rows where the part_no and contract from the sales_part table exist on HH_INV_PART_CHARS. It's not limiting the update to part where the qty_multiple isn't equal to the cases_per_layer (which is what I want).
I'm a little stumped right now. I've been trying to work on both the subqueries but I'm not having any luck identifying where the problem is.
The Regexp_like ( sp.part_no,...) in the update query refers to SALES_PART_TAB.spart_no, while in the second query it refers to SALES_PART.spart_no.
One of the causes of the fog is that you redefine the alias sp in the same query, and so the exists subquery does not relate in any way to the record that is being updated. This means that if you would throw away the exists condition, you would still update 16 records. It seems very unlikely that this is what you want.
Use a different alias, so you can distinguish which table you want to refer to.
I've created an update procedure that will change the values of a record in a table if the matching record in another table changed, however, how i've got it set up is that even though there is like 12 fields that can be updated, the procedure will update every one of them even if only 1 field has been changed
this is ofcause inefficient, it takes about 50 seconds for just 1 record, not good if there is like a few hundred, i thought up the psudocode for this easy enough
if olddata != newdata
UPDATE!!!
endif
thing is that i have little to no experience with if statements in SQL, if i could work with PHP then it would be easy by just running a SELECT with a WHERE clause much like the if statement above then checking the number of rows it returned (if 0 then there is no change) but the procedure has to be just SQL, the best i could come up with is this
IF(SELECT o.field FROM originaltbale AS o INNER JOIN updatetable AS u ON o.primarykey = u.primarykey WHERE o.field <> u.field) != 0
UPDATE!!!
ENDIF
but i doubt that'll work cause i'm sure SELECT won't return the number of rows, how can i create an if statement that'll work like this
I'm not sure I entirely understand what you're updating, but you're much better off doing this in a single statement and incorporating your IF into your WHERE clause.
UPDATE updatetable u
SET ... something ...
WHERE EXISTS ( SELECT 1 FROM originaltable o WHERE o.primarykey = u.primarykey AND ... )
But if you're storing the same field in two tables, are you sure this is the best database design for your tables?
If you want to know whether a row exists satisfying your criteria, then the EXISTS clause can be used.
-- If the original table doesn't have a record with the state same as that in updatetable
IF NOT EXISTS (SELECT o.field FROM originaltbale AS o INNER JOIN updatetable AS u ON o.primarykey = u.primarykey WHERE o.field <> u.field)
BEGIN
UPDATE !!!
END
Hope this helps.
I've got the following trigger;
CREATE TRIGGER trFLightAndDestination
ON checkin_flight
AFTER INSERT,UPDATE
AS
BEGIN
IF NOT EXISTS
(
SELECT 1
FROM Flight v
INNER JOIN Inserted AS i ON i.flightnumber = v.flightnumber
INNER JOIN checkin_destination AS ib ON ib.airport = v.airport
INNER JOIN checkin_company AS im ON im.company = v.company
WHERE i.desk = ib.desk AND i.desk = im.desk
)
BEGIN
RAISERROR('This combination of of flight and check-in desk is not possible',16,1)
ROLLBACK TRAN
END
END
What i want the trigger to do is to check the tables Flight, checkin_destination and checkin_company when a new record for checkin_flight is added. Every record of checkin_flight contains a flightnumber and desknumber where passengers need to check in for this destination.
The tables checkin_destination and checkin_company contain information about companies and destinations restricted to certain checkin desks. When adding a record to checkin_flight i need information from the flight table to get the destination and flightcompany with the inserted flightnumber. This information needs to be checked against the available checkin combinations for flights, destinations and companies.
I'm using the trigger as stated above, but when i try to insert a wrong combination the trigger allows it. What am i missing here?
EDIT 1:
I'm using the following multiple insert statement
INSERT INTO checkin_flight VALUES (5315,3),(5316,3),(5316,2)
//5315 is the flightnumber, 3 is the desknumber to checkin for that flight
EDIT 2:
Tested a single row insert which isn't possible, then the error is being thrown correct. So it's the multiple insert which seems to give the problem.
The problem is that your logic is allowing any insert that includes at least one valid set of values through. It will only fail if all of the inserted records are invalid, instead of if any of the inserted records are invalid.
Change your "IF NOT EXISTS(...)" to a statement "IF EXISTS(...)" and change your SELECT statement to return invalid flights.
eg:
IF EXISTS
(
SELECT 1
FROM Flight v
INNER JOIN Inserted AS i ON i.flightnumber = v.flightnumber
LEFT JOIN checkin_destination AS ib ON ib.airport = v.airport
AND i.desk = ib.desk
LEFT JOIN checkin_company AS im ON im.company = v.company
AND i.desk = im.desk
WHERE (im.desk IS NULL OR ib.desk IS NULL)
)
BEGIN
RAISERROR('This combination of of flight and check-in desk is not possible',16,1)
ROLLBACK TRAN
END
I'm not sure of your business logic, but you need to check that the query does the proper thing.
Your problem is the IF NOT EXISTS, if the condition is true for 1 of the 3 rows in INSERTED it does not exist. You need to convert it to find a problems row and use IF EXISTS then error out.
However, when in a trigger the best way to error out is:
RAISERROR()
ROLLBACK TRANSACTION
RETURN
I kind of doubt that the lack of a RETURN is your problem, but it is always best to include the three Rs when erroring out in a trigger.
The problem is that the condition will be true if only one of the inserted records are correct. You have to check that all records are correct, e.g.:
if (
(
select count(*) from inserted
) = (
select count(*) from flight v
inner join inserted i ...
)
) ...
The inserted table can contain multiple rows and therefore all logic within a trigger MUST be able to apply to all rows. The idea triggers must fire once per row effect is a common misunderstanding WRT triggers. SQL Server will tend to coalesce calls to a trigger to increase performance when they occur within the same transaction.
To fix you might start with a COUNT() of inserted and compare that with a COUNT() of the matching conditions and raise an error if there is a mismatch.
I've just learned ( yesterday ) to use "exists" instead of "in".
BAD
select * from table where nameid in (
select nameid from othertable where otherdesc = 'SomeDesc' )
GOOD
select * from table t where exists (
select nameid from othertable o where t.nameid = o.nameid and otherdesc = 'SomeDesc' )
And I have some questions about this:
1) The explanation as I understood was: "The reason why this is better is because only the matching values will be returned instead of building a massive list of possible results". Does that mean that while the first subquery might return 900 results the second will return only 1 ( yes or no )?
2) In the past I have had the RDBMS complainin: "only the first 1000 rows might be retrieved", this second approach would solve that problem?
3) What is the scope of the alias in the second subquery?... does the alias only lives in the parenthesis?
for example
select * from table t where exists (
select nameid from othertable o where t.nameid = o.nameid and otherdesc = 'SomeDesc' )
AND
select nameid from othertable o where t.nameid = o.nameid and otherdesc = 'SomeOtherDesc' )
That is, if I use the same alias ( o for table othertable ) In the second "exist" will it present any problem with the first exists? or are they totally independent?
Is this something Oracle only related or it is valid for most RDBMS?
Thanks a lot
It's specific to each DBMS and depends on the query optimizer. Some optimizers detect IN clause and translate it.
In all DBMSes I tested, alias is only valid inside the ( )
BTW, you can rewrite the query as:
select t.*
from table t
join othertable o on t.nameid = o.nameid
and o.otherdesc in ('SomeDesc','SomeOtherDesc');
And, to answer your questions:
Yes
Yes
Yes
You are treading into complicated territory, known as 'correlated sub-queries'. Since we don't have detailed information about your tables and the key structures, some of the answers can only be 'maybe'.
In your initial IN query, the notation would be valid whether or not OtherTable contains a column NameID (and, indeed, whether OtherDesc exists as a column in Table or OtherTable - which is not clear in any of your examples, but presumably is a column of OtherTable). This behaviour is what makes a correlated sub-query into a correlated sub-query. It is also a routine source of angst for people when they first run into it - invariably by accident. Since the SQL standard mandates the behaviour of interpreting a name in the sub-query as referring to a column in the outer query if there is no column with the relevant name in the tables mentioned in the sub-query but there is a column with the relevant name in the tables mentioned in the outer (main) query, no product that wants to claim conformance to (this bit of) the SQL standard will do anything different.
The answer to your Q1 is "it depends", but given plausible assumptions (NameID exists as a column in both tables; OtherDesc only exists in OtherTable), the results should be the same in terms of the data set returned, but may not be equivalent in terms of performance.
The answer to your Q2 is that in the past, you were using an inferior if not defective DBMS. If it supported EXISTS, then the DBMS might still complain about the cardinality of the result.
The answer to your Q3 as applied to the first EXISTS query is "t is available as an alias throughout the statement, but o is only available as an alias inside the parentheses". As applied to your second example box - with AND connecting two sub-selects (the second of which is missing the open parenthesis when I'm looking at it), then "t is available as an alias throughout the statement and refers to the same table, but there are two different aliases both labelled 'o', one for each sub-query". Note that the query might return no data if OtherDesc is unique for a given NameID value in OtherTable; otherwise, it requires two rows in OtherTable with the same NameID and the two OtherDesc values for each row in Table with that NameID value.
Oracle-specific: When you write a query using the IN clause, you're telling the rule-based optimizer that you want the inner query to drive the outer query. When you write EXISTS in a where clause, you're telling the optimizer that you want the outer query to be run first, using each value to fetch a value from the inner query. See "Difference between IN and EXISTS in subqueries".
Probably.
Alias declared inside subquery lives inside subquery. By the way, I don't think your example with 2 ANDed subqueries is valid SQL. Did you mean UNION instead of AND?
Personally I would use a join, rather than a subquery for this.
SELECT t.*
FROM yourTable t
INNER JOIN otherTable ot
ON (t.nameid = ot.nameid AND ot.otherdesc = 'SomeDesc')
It is difficult to generalize that EXISTS is always better than IN. Logically if that is the case, then SQL community would have replaced IN with EXISTS...
Also, please note that IN and EXISTS are not same, the results may be different when you use the two...
With IN, usually its a Full Table Scan of the inner table once without removing NULLs (so if you have NULLs in your inner table, IN will not remove NULLS by default)... While EXISTS removes NULL and in case of correlated subquery, it runs inner query for every row from outer query.
Assuming there are no NULLS and its a simple query (with no correlation), EXIST might perform better if the row you are finding is not the last row. If it happens to be the last row, EXISTS may need to scan till the end like IN.. so similar performance...
But IN and EXISTS are not interchangeable...