I'm working through this exercise.
On question 4, the goal is to find employees hired after "Jones". I think this problem can be solved without a join like so:
SELECT first_name, last_name, hire_date
FROM employees
WHERE hire_date > (
SELECT hire_date FROM employees WHERE last_name = "Jones"
)
But the answer on the website suggests:
SELECT e.first_name, e.last_name, e.hire_date
FROM employees e
JOIN employees davies
ON (davies.last_name = "Jones")
WHERE davies.hire_date < e.hire_date;
Are these more-or-less the same or is there a reason the second answer should be considered better?
I assume that the column last_name is defined as UNIQUE, so that the subquery in the 1st query returns only 1 row.
If not, then the queries do not return the same results, because although the subquery in the 1st query may return more than 1 row (and in other databases the query would not even run), SQLite will pick just the 1st of the returned rows and use its hire_date to compare it with all the rows of the table, while the join will use all the rows where last_name = "Jones".
If my assumption is correct then the 2 queries are equivalent, but the 1st one is what I would suggest because it is more readable and I believe it would perform better than the join.
If I had to use a join for this requirement (since it is homework) I would choose a more readable form:
SELECT e.first_name, e.last_name, e.hire_date
FROM employees e
JOIN (SELECT * FROM employees WHERE last_name = "Jones") t
ON t.hire_date < e.hire_date;
I'm trying to write a query that in Teradata but I'm not sure how to do it; my table looks like this:
col1: text (account_number)
col2: text (secondary account number)
col3: text (Primary_cust)
the business requirements are:
"Group records by account number.
If there is only one record for an account then keep that record.
If there are multiple records for an account number then:
(1) if only one record has Primary_CUST = 'Y' then keep.
(2) if multiple records have Primary_CUST = 'Y' then keep one with lowest SCDRY_ACCT_NBR
(3) If no records have Primary_CUST = 'Y' then keep one with lowest SCDRY_ACCT_NBR.
I know I need a CASE statement and I'm able to write the first requirement, but not sure on the second. Any help would be greatly appreciated.
You just have to think about how to order the rows to get the row you want on top, seems to be like this:
SELECT * FROM tab
QUALIFY
Row_Number()
Over (PARTITION BY account_number -- for each account
ORDER BY Primary_CUST DESC -- 'Y' before 'N' (assuming it's a Y/N column)
,SCDRY_ACCT_NBR -- lowest number
) = 1 -- return the top row
Of course QUALIFY is proprietary Teradata syntax, if you need to do this on Oracle you have to wrap it in a Derived Table:
SELECT *
FROM
(
SELECT t.*,
Row_Number()
Over (PARTITION BY account_number -- for each account
ORDER BY Primary_CUST DESC -- 'Y' before 'N' (assuming it's a Y/N column)
,SCDRY_ACCT_NBR) AS rn-- lowest number
FROM tab
) AS dt
WHERE rn = 1 -- return the top row
Can some one please help in solving my problem
I have three tables to be joined ed using indexes in Teradata to improve performance. Query specified below:-
Select b.Id, b.First_name, b.Last_name, c. Id,
c.First_name, c.Last_name, c.Result
from
(
select a.Id, a.First_name, a. Last_name, a.Approver1, a.Approver2
From table1 a
Inner join table2 d
On a.Id =D.Id
and A.Approver1 =a.Approver1
And a.Approve2 =D.Approver2
) b
Left join
(
select * from table3
where result is not null
and application like 'application1'
) c
On c. Id=b.Id
Group by b.Id, b.First_name, b.Last_name, c.Id,
c.First_name, c.Last_name, c.Result
The above query is taking so much of time since PI not defined correctly.
First two tables (table1 and 2) are with same set of columns hence pi can be defined like PI on I'd, approve1, approve2
However, while joining with table3 am confused and need to understand how to define pi. Is it something that PI can only work when we have same set of columns in the tables?
Structure of table3 is
I'd, first name, last name, result
And table 1 and table2
Id , First name, Last name, Approved 1, Approved 2, Results
Can you please help in defining primary indexes so that query can be optimised.
Teradata will usually not use Secondary Indexes for joins. The best PI would be id for all three tables, of course you need to check if there are not too many rows per value and it's not too skewed.
GROUP BY can be simplified to a DISTINCT, why do you need it, can you show the Primary Keys of those tables?
Edit based on comment:
PI-based joins are by far the fastest way. But you should be able the get rid of the DISTINCT, too, it's always a huge overhead.
Try replacing the 1st join with a NOT EXISTS:
Select b.Id, b.First_name, b.Last_name, c. Id,
c.First_name, c.Last_name, c.Result
from
(
select a.Id, a.First_name, a. Last_name, a.Approver1, a.Approver2
From table1 a
WHERE EXISTS
(
SELECT *
FROM table2 d
WHERE a.Id =D.Id
and A.Approver1 =a.Approver1
And a.Approve2 =D.Approver2
)
) b
Left join
(
select * from table3
where result is not null
and application like 'application1'
) c
On c. Id=b.Id
I would like to know how to make a single query to select something from one table or from another. For example I have Table A with the aID=2 and Table B with bID=3. So how to make the query to return the id from table A or table B for a given id = 3?
Thanks in advance
If you do not know which table to use before you're running the query, you can combine multiple queries with UNION, provided that you're selecting the same number of columns:
SELECT something FROM A WHERE aID = 3
UNION ALL
SELECT something FROM B WHERE bID = 3
If you do not want to duplicate the WHERE condition, you can use a nested query and apply the WHERE to its result:
SELECT something
FROM (SELECT aID as ID, something FROM A
UNION ALL
SELECT bID , something FROM B)
WHERE ID = 3
I am trying to update Table B of a database looking like this:
Table A:
id, amount, date, b_id
1,200,6/31/2012,1
2,300,6/31/2012,1
3,400,6/29/2012,2
4,200,6/31/2012,1
5,200,6/31/2012,2
6,200,6/31/2012,1
7,200,6/31/2012,2
8,200,6/31/2012,2
Table B:
id, b_amount, b_date
1,0,0
2,0,0
3,0,0
Now with this query I get all the data I need in one select:
SELECT A.*,B.* FROM A LEFT JOIN B ON B.id=A.b_id WHERE A.b_id>0 GROUP BY B.id
id, amount, date, b_id, id, b_amount, b_date
1,200,6/31/2012,1,1,0,0
3,400,6/29/2012,1,1,0,0
Now, I just want to copy the selected column amount to b_amount and date to b_date
b_amount=amount, b_date=date
resulting in
id, amount, date, b_id, id, b_amount, b_date
1,200,6/31/2012,1,1,200,6/31/2012
3,400,6/29/2012,1,1,400,6/29/2012
I've tried COALESCE() without success.
Does someone experienced have a solution for this?
Solution:
Thanks to the answers below, I managed to come up with this. It is probably not the most efficient way but it is fine for a one time only update. This will insert for you the first corresponding entry of each group.
REPLACE INTO A SELECT id, amount, date FROM
(SELECT A.id, A.amount, B.id as Bid FROM A INNER JOIN B ON (B.id=A.B_id)
ORDER BY A.id DESC)
GROUP BY Bid;
So what you are looking for seems to be a JOIN inside of an UPDATE query. In mySQL you would use
UPDATE B INNER JOIN A ON B.id=A.b_id SET B.amount=A.amount, B.date=A.date;
but this is not supported by sqlite as this probably related question points out. However, there is a workaround using REPLACE:
REPLACE INTO B
SELECT B.id, A.amount, A.date FROM A
LEFT JOIN B ON B.id=A.b_id
WHERE A.b_id>0 GROUP BY B.id;
The query will simply fill in the values of table B for all columns which should keep their state and fill in the values of table A for the copied values. Make sure the order of the columns in the SELECT statement meet your column order of table B and all columns are mentioned or you will loose these field's data. This is probably dangerous for future changes on table B. So keep in mind to change the column order/presence of this query when changing table B.
Something a bit off topic, because you did not ask for that: A.b_id is obviously a foreign key to B.id. It seems you are using the value 0 for the foreign key to express that there is no corresponding entry in B. (Inferred from your SELECT with WHERE A.b_id>0.) You should consider using the null value for that. When you are using INNER JOIN then instead of LEFT JOIN you can drop the WHERE clause entirely. The DBS will then sort out all unsatisfied relations.
WARNING Some RDBMS will return 2 rows as you show above. Others will return the Cartesian product of the rows i.e. A rows times B rows.
One tricky method is to generate SQL that is then executed
SELECT "update B set b.b_amount = ", a.amount, ", b.b_date = ", a.date,
" where b.id = ", a.b_id
FROM A LEFT JOIN B ON B.id=A.b_id WHERE A.b_id>0 GROUP BY B.id
Now add the batch terminator and execute this SQL. The query result should look like this
update B set b.b_amount = 200, b.b_date = 6/31/2012 where b.id = 1
update B set b.b_amount = 400, b.b_date = 6/29/2012 where b.id = 3
NOTE: Some RDBMS will handle dates differently. Some require quotes.