Hi all when I executed this query somehow its throwing the following error -
ORA-00907: missing right parenthesis.
But if you remove the order by 1 from SELECT 2 FROM DUAL order by 1 its working.
Did I miss something out here or its ORACLE limitation
SELECT (CASE
WHEN EXISTS
(SELECT 1 FROM DUAL) THEN
(SELECT 4
FROM dual)
ELSE
(SELECT 2 FROM DUAL order by 1 )
END) AS DELEGATOR FROM dual
Below is a working code with order by 1 removed
SELECT (CASE
WHEN EXISTS
(SELECT 1 FROM DUAL) THEN
(SELECT 4
FROM dual)
ELSE
(SELECT 2 FROM DUAL )
END) AS DELEGATOR FROM dual
Somehow I already give up but when I change the code to this
it somehow works. I applied to my actual query and the result
are the expected outcome.
SELECT (CASE
WHEN EXISTS
(SELECT 1 FROM DUAL) THEN
(SELECT 4
FROM dual)
ELSE
(select * from (SELECT 2 FROM DUAL order by 1 )
where rownum = 1)
END) AS DELEGATOR FROM dual
SELECT 2 FROM DUAL order by 1
Firstly, an ORDER BY makes no sense here the query returns just one row.
Secondly, you cannot use an order by in a Scalar Subquery Expression when there is no sense of ordering the rows. Order by would be allowed in certain cases where the outer query expects an ordered set of rows. In your case, there is no question of getting more than a single-row.
Thirdly, it is not a good practice coding as order by 1,2 etc. Instead, use the appropriate column name. You might not come to know if the columns in the select list changes, your result set would be then ordered differently and you will need to make changes at both the places.
A scalar subquery expression returns exactly one column value from one row. If the expression returned more than one row you would get an error, "ORA-01427: single-row subquery returns more than one row". As it can only have a single value, ordering that value would be meaningless.
The missing-right-parenthesis error doesn't necessarily mean you have unbalanced parentheses, it can indicate other errors that have made the parser give up at the point it expected to see one. Here the parser expected the subquery's closing parenthesis to have come after the FROM DUAL, so it's stopping when it doesn't see one - effectively it doesn't know how to interpret the rest of the statement so it doesn't try (hugely oversimplifying).
An order by clause isn't meaningful in a subquery, and generally isn't allowed; though it is tolerated in some places, such as an inline view, despite still having no effect.
Obviously your example is very contrived, but there is a scenario where you might think you want an order by clause, and that is to get the first value from a result set which you need to be ordered. Based on how limit works in other databases you might try to do something like:
select (
select object_name from user_objects
where rownum = 1
order by created desc
)
from dual
... but that isn't how rownum works, and would also get ORA-00907. If that is what you're doing you would need another layer of subquery (as an inline view now) which can be ordered, and apply the rownum filter to that:
select (
select object_name from (
select object_name from user_objects
order by created desc
)
where rownum = 1
)
from dual
... which is now valid.
(Whether you actually need to be doing that in a subquery, rather than through a join, is another matter - your query is hopefully complicated enough to warrant it).
I found this example code online when I searched for "how to do an Exclusive Between oracle sql"
Someone was proving that, in Oracle, BETWEEN is by default inclusive.
So they used such code :
with x as (
select 1 col1 from dual
union
select 2 col1 from dual
union
select 3 col1 from dual
UNION
select 4 col1 from dual
)
select *
from x
where col1 between 2 and 3
I've never seen such an example, what is going on with the WITH ?
In short, WITH clause is an inline view, or subquery. It is useful when you will refer to something multiple times, or when you want to abstract parts of a complex query to make it easier to read.
If you are from SQL Server world, you can also think of it like a temporary table.
So:
WITH foo as (select * from tab);
select * from foo;
is like
select * from (select * from tab);
Though it may be more efficient since x is resolved to a single dataset, even if queried multiple times.
It also reduces repetition. If you use a subquery more than once in a statement, you can consider factoring it out using WITH.
It has nothing to do with the BETWEEN example, it is just the author's choice of approach for demonstrating a concept.
Here's my query:
SELECT my_view.*
FROM my_view
WHERE my_view.trial in (select 2 as trial_id from dual union select 3 from dual union select 4 from dual)
and my_view.location like ('123-%')
When I execute this query it returns results which do not conform to the my_view.location like ('123-%') condition. It's as if that condition is being ignored completely. I can even change it to my_view.location IS NULL and it returns the same results, despite that field being not-nullable.
I know this query seems ridiculous with the selects from dual, but I've structured it this way to replicate a problem I have when I use a 'WITH' clause (the results of that query are where the selects from dual inline view are).
I can modify the query like so and it returns the expected results:
SELECT my_view.*
FROM my_view
WHERE my_view.trial in (2, 3, 4)
and my_view.location like ('123-%')
Unfortunately I do not know the trial values up front (they are queried for in a 'WITH' clause) so I cannot structure my query this way. What am I doing wrong?
I will say that the my_view view is composed of 3 other views whose results are UNION ALL and each of which retrieve some data over a DB Link. Not that I believe that should matter, but in case it does.
One thing you could try if you don't have luck with this route is to replace "IN" with an "EXISTS" or "NOT EXISTS" statement.
If you could accomplish what you want using joins, that would be the best option because of performance. If you have views pulling data from views, you can often make a single query to do what you want that gives you better performance using subqueries.
If you do
EXPLAIN PLAN FOR
SELECT my_view.*
FROM my_view
WHERE my_view.trial in (select 2 as trial_id from dual union select 3 from dual union select 4 from dual)
and my_view.location like ('123-%');
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);
you should see where the location predicate is 9or is not) being applied. My bet is that it has something to do with the DB links and you won't be able to reproduce it if all the tables are local.
Optimizing a distributed query gets complicated.
Try changing the UNION query to use UNION ALL, as in:
SELECT my_view.*
FROM my_view
WHERE my_view.trial in (select 2 as trial_id from dual
UNION ALL
select 3 AS TRIAL_ID from dual
UNION ALL
select 4 AS TRIAL_ID from dual)
and my_view.location like ('123-%')
I also put in "AS TRIAL_ID" on the 3 and 4 cases. I agree that neither of these should matter, but I've run into cases occasionally where things that I thought shouldn't matter mattered.
Good luck.
What is the difference between UNION and UNION ALL?
UNION removes duplicate records (where all columns in the results are the same), UNION ALL does not.
There is a performance hit when using UNION instead of UNION ALL, since the database server must do additional work to remove the duplicate rows, but usually you do not want the duplicates (especially when developing reports).
To identify duplicates, records must be comparable types as well as compatible types. This will depend on the SQL system. For example the system may truncate all long text fields to make short text fields for comparison (MS Jet), or may refuse to compare binary fields (ORACLE)
UNION Example:
SELECT 'foo' AS bar UNION SELECT 'foo' AS bar
Result:
+-----+
| bar |
+-----+
| foo |
+-----+
1 row in set (0.00 sec)
UNION ALL example:
SELECT 'foo' AS bar UNION ALL SELECT 'foo' AS bar
Result:
+-----+
| bar |
+-----+
| foo |
| foo |
+-----+
2 rows in set (0.00 sec)
Both UNION and UNION ALL concatenate the result of two different SQLs. They differ in the way they handle duplicates.
UNION performs a DISTINCT on the result set, eliminating any duplicate rows.
UNION ALL does not remove duplicates, and it therefore faster than UNION.
Note: While using this commands all selected columns need to be of the same data type.
Example: If we have two tables, 1) Employee and 2) Customer
Employee table data:
Customer table data:
UNION Example (It removes all duplicate records):
UNION ALL Example (It just concatenate records, not eliminate duplicates, so it is faster than UNION):
UNION removes duplicates, whereas UNION ALL does not.
In order to remove duplicates the result set must be sorted, and this may have an impact on the performance of the UNION, depending on the volume of data being sorted, and the settings of various RDBMS parameters ( For Oracle PGA_AGGREGATE_TARGET with WORKAREA_SIZE_POLICY=AUTO or SORT_AREA_SIZE and SOR_AREA_RETAINED_SIZE if WORKAREA_SIZE_POLICY=MANUAL ).
Basically, the sort is faster if it can be carried out in memory, but the same caveat about the volume of data applies.
Of course, if you need data returned without duplicates then you must use UNION, depending on the source of your data.
I would have commented on the first post to qualify the "is much less performant" comment, but have insufficient reputation (points) to do so.
In ORACLE: UNION does not support BLOB (or CLOB) column types, UNION ALL does.
The basic difference between UNION and UNION ALL is union operation eliminates the duplicated rows from the result set but union all returns all rows after joining.
from http://zengin.wordpress.com/2007/07/31/union-vs-union-all/
UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type. With UNION, only distinct values are selected.
UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.
The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines them into a table.
A UNION statement effectively does a SELECT DISTINCT on the results set. If you know that all the records returned are unique from your union, use UNION ALL instead, it gives faster results.
You can avoid duplicates and still run much faster than UNION DISTINCT (which is actually same as UNION) by running query like this:
SELECT * FROM mytable WHERE a=X UNION ALL SELECT * FROM mytable WHERE b=Y AND a!=X
Notice the AND a!=X part. This is much faster then UNION.
Just to add my two cents to the discussion here: one could understand the UNION operator as a pure, SET-oriented UNION - e.g. set A={2,4,6,8}, set B={1,2,3,4}, A UNION B = {1,2,3,4,6,8}
When dealing with sets, you would not want numbers 2 and 4 appearing twice, as an element either is or is not in a set.
In the world of SQL, though, you might want to see all the elements from the two sets together in one "bag" {2,4,6,8,1,2,3,4}. And for this purpose T-SQL offers the operator UNION ALL.
UNION - results in distinct records while
UNION ALL - results in all the records including duplicates.
Both are blocking operators and hence I personally prefer using JOINS over Blocking Operators(UNION, INTERSECT, UNION ALL etc. ) anytime.
To illustrate why Union operation performs poorly in comparison to Union All checkout the following example.
CREATE TABLE #T1 (data VARCHAR(10))
INSERT INTO #T1
SELECT 'abc'
UNION ALL
SELECT 'bcd'
UNION ALL
SELECT 'cde'
UNION ALL
SELECT 'def'
UNION ALL
SELECT 'efg'
CREATE TABLE #T2 (data VARCHAR(10))
INSERT INTO #T2
SELECT 'abc'
UNION ALL
SELECT 'cde'
UNION ALL
SELECT 'efg'
Following are results of UNION ALL and UNION operations.
A UNION statement effectively does a SELECT DISTINCT on the results set. If you know that all the records returned are unique from your union, use UNION ALL instead, it gives faster results.
Using UNION results in Distinct Sort operations in the Execution Plan. Proof to prove this statement is shown below:
Not sure that it matters which database
UNION and UNION ALL should work on all SQL Servers.
You should avoid of unnecessary UNIONs they are huge performance leak. As a rule of thumb use UNION ALL if you are not sure which to use.
(From Microsoft SQL Server Book Online)
UNION [ALL]
Specifies that multiple result sets are to be combined and returned as a single result set.
ALL
Incorporates all rows into the results. This includes duplicates. If not specified, duplicate rows are removed.
UNION will take too long as a duplicate rows finding like DISTINCT is applied on the results.
SELECT * FROM Table1
UNION
SELECT * FROM Table2
is equivalent of:
SELECT DISTINCT * FROM (
SELECT * FROM Table1
UNION ALL
SELECT * FROM Table2) DT
A side effect of applying DISTINCT over results is a sorting operation on results.
UNION ALL results will be shown as arbitrary order on results But UNION results will be shown as ORDER BY 1, 2, 3, ..., n (n = column number of Tables) applied on results. You can see this side effect when you don't have any duplicate row.
I add an example,
UNION, it is merging with distinct --> slower, because it need comparing (In Oracle SQL developer, choose query, press F10 to see cost analysis).
UNION ALL, it is merging without distinct --> faster.
SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual
UNION
SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual;
and
SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual
UNION ALL
SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual;
UNION merges the contents of two structurally-compatible tables into a single combined table.
Difference:
The difference between UNION and UNION ALL is that UNION will omit duplicate records whereas UNION ALL will include duplicate records.
Union Result set is sorted in ascending order whereas UNION ALL Result set is not sorted
UNION performs a DISTINCT on its Result set so it will eliminate any duplicate rows. Whereas UNION ALL won't remove duplicates and therefore it is faster than UNION.*
Note: The performance of UNION ALL will typically be better than UNION, since UNION requires the server to do the additional work of removing any duplicates. So, in cases where it is certain that there will not be any duplicates, or where having duplicates is not a problem, use of UNION ALL would be recommended for performance reasons.
Suppose that you have two table Teacher & Student
Both have 4 Column with different Name like this
Teacher - ID(int), Name(varchar(50)), Address(varchar(50)), PositionID(varchar(50))
Student- ID(int), Name(varchar(50)), Email(varchar(50)), PositionID(int)
You can apply UNION or UNION ALL for those two table which have same number of columns. But they have different name or data type.
When you apply UNION operation on 2 tables, it neglects all duplicate entries(all columns value of row in a table is same of another table). Like this
SELECT * FROM Student
UNION
SELECT * FROM Teacher
the result will be
When you apply UNION ALL operation on 2 tables, it returns all entries with duplicate(if there is any difference between any column value of a row in 2 tables). Like this
SELECT * FROM Student
UNION ALL
SELECT * FROM Teacher
Output
Performance:
Obviously UNION ALL performance is better that UNION as they do additional task to remove the duplicate values. You can check that from Execution Estimated Time by press ctrl+L at MSSQL
UNION removes duplicate records in other hand UNION ALL does not. But one need to check the bulk of data that is going to be processed and the column and data type must be same.
since union internally uses "distinct" behavior to select the rows hence it is more costly in terms of time and performance.
like
select project_id from t_project
union
select project_id from t_project_contact
this gives me 2020 records
on other hand
select project_id from t_project
union all
select project_id from t_project_contact
gives me more than 17402 rows
on precedence perspective both has same precedence.
If there is no ORDER BY, a UNION ALL may bring rows back as it goes, whereas a UNION would make you wait until the very end of the query before giving you the whole result set at once. This can make a difference in a time-out situation - a UNION ALL keeps the connection alive, as it were.
So if you have a time-out issue, and there's no sorting, and duplicates aren't an issue, UNION ALL may be rather helpful.
One more thing i would like to add-
Union:- Result set is sorted in ascending order.
Union All:- Result set is not sorted. two Query output just gets appended.
Important! Difference between Oracle and Mysql: Let's say that t1 t2 don't have duplicate rows between them but they have duplicate rows individual. Example: t1 has sales from 2017 and t2 from 2018
SELECT T1.YEAR, T1.PRODUCT FROM T1
UNION ALL
SELECT T2.YEAR, T2.PRODUCT FROM T2
In ORACLE UNION ALL fetches all rows from both tables. The same will occur in MySQL.
However:
SELECT T1.YEAR, T1.PRODUCT FROM T1
UNION
SELECT T2.YEAR, T2.PRODUCT FROM T2
In ORACLE, UNION fetches all rows from both tables because there are no duplicate values between t1 and t2. On the other hand in MySQL the resultset will have fewer rows because there will be duplicate rows within table t1 and also within table t2!
UNION ALL also works on more data types as well. For example when trying to union spatial data types. For example:
select a.SHAPE from tableA a
union
select b.SHAPE from tableB b
will throw
The data type geometry cannot be used as an operand to the UNION, INTERSECT or EXCEPT operators because it is not comparable.
However union all will not.