How to create a view from multiple tables? - oracle

CREATE VIEW Te AS
SELECT
select sno,sname,dept,'madinah'as universty name from med_std
union
select sno,sname,dept,'yanbu'as universty name from yun_std

You have two SELECT statements in a row; remove one of them:
CREATE VIEW Te AS
select sno, sname, dept, 'madinah' as universty name from med_std
union
select sno, sname, dept, 'yanbu' as universty name from yun_std
Whilst you're at it you may want to seriously consider normalising your database; why does every university have a separate table? If you put it all in one table then you don't need to query multiple tables.
Furthermore, I'd highly recommend using UNION ALL in this situation if at all possible, instead of UNION. UNION will attempt to do a distinct sort on the result set; as your university's have different names between the two tables there is no need to do a distinct so you might just as well not attempt it. You should only use UNION if you want to remove duplicates from within one of your tables. See the documentation for more information.

Related

Android Room Complex Queries

All the examples I've seen show a simple selection "SELECT * from user". But we all know in the real world we will have queries like this:
SELECT COUNT(1) QTD_MY_PENDING_SO FROM ( SELECT IFNULL((SELECT count(1) FROM (SELECT s.customer_code, s.so_prefix, ... continuing with many lines.
That's the reason why we use rawQuery from DB on SQLite to select columns from many tables.
So, my question is, how do we make complex queries that will return columns from many tables in Room?

Oracle - select statement alias one column and wildcard to get all remaining columns

New to SQL. Pardon me if this question is a basic one. Is there a way for me to do this below
SELECT COLUMN1 as CUSTOM_NAME, <wildcard for remaining columns as is> from TABLE;
I only want COLUMN1 appear once in the final result
There is no way to make that kind of dynamic SELECT list with regular SQL*.
This is a good thing. Programming gets more difficult the more dynamic it is. Even the simple * syntax, while useful in many contexts, causes problems in production code. The Oracle SQL grammar is already more complicated than most traditional programming languages, adding a little meta language to describe what the queries return could be a nightmare.
*Well, you could create something using Oracle data cartridge, or DBMS_XMLGEN, or a trick with the PIVOT clause. But each of those solutions would be incredibly complicated and certainly not as simple as just typing the columns.
This is about as close as you will get.
It is very handy for putting the important columns up front,
while being able to scroll to the others if needed. COLUMN1 will end up being there twice.
SELECT COLUMN1 as CUSTOM_NAME,
aliasName.*
FROM TABLE aliasName;
In case you have many columns it might be worth to generate a full column list automatically instead of relying on the * selector.
So a two step approach would be to generate the column list with custom first N columns and unspecified order of the other columns, then use this generated list in your actual select statement.
-- select comma separated column names from table with the first columns being in specified order
select
LISTAGG(column_name, ', ') WITHIN GROUP (
ORDER BY decode(column_name,
'FIRST_COLUMN_NAME', 1,
'SECOND_COLUMN_NAME', 2) asc) "Columns"
from user_tab_columns
where table_name = 'TABLE_NAME';
Replace TABLE_NAME, FIRST_COLUMN_NAME and SECOND_COLUMN_NAME by your actual names, adjust the list of explicit columns as needed.
Then execute the query and use the result, which should look like
FIRST_COLUMN_NAME, SECOND_COLUMN_NAME, OTHER_COLUMN_NAMES
Ofcourse this is overhead for 5-ish columns, but if you ever run into a company database with 3 digit number of columns, this can be interesting.

ORA-00979 not a Group By function error

Iam trying to select 2 values from a Table, Employee emp_name, emp_location grouping by emp_location, iam aware that the columns which are in group by function needs to be in select clause, but i would like to know whether is there any other way to get these value in a single query.
My intention is to select only one employee per location based on age.
sample query
select emp_name,emp_location
from Employee
where emp_age=25
group by emp_location
please help in this regard.
Thanks a lot for all the guys who have responded for this question. I will try to learn these windows functions as these are very handy.
The reason why this works in MySQL and not in Oracle, is because in Oracle, as well most other databases, you either need to specify a field (or expression) in the group by clause, or it has to be an aggregation which combines the values of all values in the group into a single one. For instance, this would work:
select max(emp_name),emp_location
from Employee
where emp_age=25
group by emp_location
However, it's may not the best solution. It will work if you want just the name, but you'll get into trouble when you want to have multiple fields for an employee. In that case max won't do the trick. In the query below, you might get a first name that doesn't match the last name.
select max(emp_firstname), max(emp_lastname), emp_location
from Employee
where emp_age=25
group by emp_location
On solution for this, is using a window function (analytical function). With those, you can generate a value for each record, without immediately reducing the number of records. For instance, with a windowed max function, you could select the max age for people named John, and display that value next to every John in the result, even if they don't have that age.
Some functions, like rank, dense_rank and row_number can be used to generate a number for each employee, which you can then use to filter by. In the example below, I created such a counter per location (partition by), and ordered by, in this case name and id. You can specify other fields as well, for instance if you want one name per age per location, you specify both age and location in partition by. If you want the oldest employee of each location, you can remove where emp_age=25 and order by emp_age desc instead.
select
*
from
(select
emp_name, emp_location,
dense_rank() over (partition by emp_location order by emp_name, emp_id) as emp_rank
from Employee
where emp_age=25)
where
emp_rank = 1
ORA-00979 not a Group By function error
Only aggregate functions and columns specified in the GROUP BY clause are allowed in the SELECT clause.
In that regard, Oracle follows the SQL standard closely. But, as you noticed in your comment, some other RDBMS are less strict than Oracle regarding that point. For example, to quote MySQL's documentation (emphasis mine):
MySQL extends the use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause. [...]
However, this is useful primarily when all values in each nonaggregated column not named in the GROUP BY are the same for each group. The server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate.
So, in the recommended use case, adding the extra columns to the GROUP BY clause will lead to the same result.
select emp_name,emp_location
-- ^^^^^^^^
-- this is *not* part of the ̀`GROUP BY` clause
from Employee
where emp_state=25
group by emp_location
Maybe are you looking for:
...
group by emp_location, emp_name
select emp_name,emp_location
from Employee
where emp_age=25
group by emp_name,emp_location
or
select max(emp_name) emp_name,emp_location
from Employee
where emp_age=25
group by emp_location

Oracle Select Query, Order By + Limit Results

I am new to Oracle and working with a fairly large database. I would like to perform a query that will select the desired columns, order by a certain column and also limit the results. According to everything I have read, the below query should be working but it is returning "ORA-00918: column ambiguously defined":
SELECT * FROM(SELECT * FROM EAI.EAI_EVENT_LOG e,
EAI.EAI_EVENT_LOG_MESSAGE e1 WHERE e.SOURCE_URL LIKE '%.XML'
ORDER BY e.REQUEST_DATE_TIME DESC) WHERE ROWNUM <= 20
Any suggestions would be greatly appreciated :D
The error message means your result set contains two columns with the same name. Each column in a query's projection needs to have a unique name. Presumably you have a column (or columns) with the same name in both EAI_EVENT_LOG and EAI_EVENT_LOG_MESSAGE.
You also want to join on that column. At the moment you are generating a cross join between the two tables. In other words, if you have a hundred records in EAI_EVENT_LOG and two hundred records EAI_EVENT_LOG_MESSAGE your result set will be twenty thousand records (without the rownum). This is probably your intention.
"By switching to innerjoin, will that eliminate the error with the
current code?"
No, you'll still need to handle having two columns with the same name. Basically this comes from using SELECT * on two multiple tables. SELECT * is bad practice. It's convenient but it is always better to specify the exact columns you want in the query's projection. That way you can include (say) e.TRANSACTION_ID and exclude e1.TRANSACTION_ID, and avoid the ORA-00918 exception.
Maybe you have some columns in both EAI_EVENT_LOG and EAI_EVENT_LOG_MESSAGE tables having identical names? Instead of SELECT * list all columns you want to select.
Other problem I see is that you are selecting from two tables but you're not joining them in the WHERE clause hence the result set will be the cross product of those two table.
You need to stop using SQL '89 implicit join syntax.
Not because it doesn't work, but because it is evil.
Right now you have a cross join which in 99,9% of the cases is not what you want.
Also every sub-select needs to have it's own alias.
SELECT * FROM
(SELECT e.*, e1.* FROM EAI.EAI_EVENT_LOG e
INNER JOIN EAI.EAI_EVENT_LOG_MESSAGE e1 on (......)
WHERE e.SOURCE_URL LIKE '%.XML'
ORDER BY e.REQUEST_DATE_TIME DESC) s WHERE ROWNUM <= 20
Please specify a join criterion on the dotted line.
Normally you do a join on a keyfield e.g. ON (e.id = e1.event_id)
It's bad idea to use select *, it's better to specify exactly which fields you want:
SELECT e.field1 as customer_id
,e.field2 as customer_name
.....

oracle-inline view

Why inline views are used..??
There are many different reasons for using inline views. Some things can't be done without inline views, for example:
1) Filtering on the results of an analytic function:
select ename from
( select ename, rank() over (order by sal desc) rnk
from emp
)
where rnk < 4;
2) Using ROWNUM on ordered results:
select ename, ROWNUM from
( select ename
from emp
order by ename
);
Other times they just make it easier to write the SQL you want to write.
The inline view is a construct in Oracle SQL where you can place a query in the SQL FROM, clause, just as if the query was a table name.
Inline views provide
Bind variables can be introduced inside the statement to limit the data
Better control over the tuning
Visibility into the code
To get top N ordered rows.
SELECT name, salary,
FROM (SELECT name, salary
FROM emp
ORDER BY salary DESC)
WHERE rownum <= 10;
An inline view can be regarded as an intermediate result set that contributes to the required data set in some way. Sometimes it is entirely a matter of improving maintainability of the code, and sometimes it is logically neccessary.
From the Oracle Database Concepts document there are the inline view concept definition:
An inline view is not a schema object.
It is a subquery with an alias
(correlation name) that you can use
like a view within a SQL statement.
About the subqueries look in Using Subqueries from the Oracle SQL Reference manual. It have a very nice pedagogic information.
Anyway, today is preferred to use the Subquery Factoring Clause that is a more powerfull way of use inline views.
As an example of all together:
WITH
dept_costs AS (
SELECT department_name, SUM(salary) dept_total
FROM employees e, departments d
WHERE e.department_id = d.department_id
GROUP BY department_name),
avg_cost AS
SELECT * FROM dept_costs
WHERE dept_total >
(SELECT avg FROM (SELECT SUM(dept_total)/COUNT(*) avg
FROM dept_costs)
)
ORDER BY department_name;
There you can see one of all:
An inline view query: SELECT SUM...
A correlated subquery: SELECT avg FROM...
A subquery factoring: dept_costs AS (...
What are they used for?:
To avoid creating an intermediate view object: CREATE VIEW ...
To simplify some queries that a view cannot be helpfull. For instance, when the view need to filter from the main query.
You will often use inline views to break your query up into logical parts which helps both readability and makes writing more complex queries a bit easier.
Jva and Tony Andrews provided some good examples of simple cases where this is useful such as Top-N or Pagination queries where you may want to perform a query and order its results before using that as a part of a larger query which in turn might feed a query doing some other processing, where the logic for these individual queries would be difficult to achieve in a single query.
Another case they can be very useful is if you are writing a query that joins various tables together and want to perform aggregation on some of the tables, separating group functions and the processing into different inline views before performing the joins makes managing cardinality a lot easier. If you want some examples, I would be happy to provide them to make it more clear.
Factored subqueries (where you list your queries in the WITH clause at the start of the query) and inline views also often bring performance benefits. If you need to access the results of the subquery multiple times, you only need to run it once and it can be materialized as a Global Temporary Table (how the optimizer acts isn't totally black and white so I won't go into it here but you can do your own research - for example, see http://jonathanlewis.wordpress.com/2007/07/26/subquery-factoring-2/)

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