Can I write the T-SQL like below
select *
FROM (select *
from TableA
where FieldA = 1
)
where FieldB > 10
which means I want to query from the results of another query.
Yes you can
select *
FROM ( select * from TableA where FieldA=1 ) sub
where FieldB > 10
Just remember to give the sub select an alias.
Yes, you can do that.
If you want to separate out your sub-queries you can also use Common Table Expressions (CTE's) which help make your code more readable:
WITH Foo (FieldA, FieldB, FieldC) AS
(
SELECT FieldA, FieldB, FieldC
FROM TableA
WHERE FieldA=1
)
SELECT *
FROM Foo
WHERE FieldB > 10
The downside is you will have to explicitly name your columns. However, this actually makes your code faster so it's not always a bad thing.
Related
I'm having a bit of an issue with some derived tables that I hope someone will be able to help with. What I've got is 2 derived tables inside a select statement that then uses a pivot to display the results horizontally rather than vertically.
What I've got so far is:
SELECT * FROM(
SELECT SUM(Value) AS TotDays, ClassId FROM MainTable GROUP BY ClassId)
Union All
SELECT SUM(NumDays) As TotDays, ClassId FROM (
SELECT CASE WHEN COUNT(SiteId) > 0 THEN 1 ELSE 0 END AS NumDays
FROM Table2 GROUP BY ClassId ) as SUB
) AS a
PIVOT (SUM(TotDays) FROM ClassId
IN ([12],[13],[14],[15]
What I'm trying to do is reference the individual columns rather than using SELECT *, but I don't know how to do it. I can make it work without if I drop everything from the union onwards, but when I put the union in it doesn't work and I have to use SELECT *.
Anyone got any ideas on what's going wrong?
Thanks
Alex
You have a couple of errors on your query. For example, your UNION ALL has sets with a different number of columns, and you have other syntax errors. Try this way:
SELECT [12],[13],[14],[15]
FROM ( SELECT SUM(Value) AS TotDays, ClassId
FROM MainTable
GROUP BY ClassId
UNION ALL
SELECT SUM(NumDays) As TotDays, ClassId
FROM ( SELECT CASE WHEN COUNT(SiteId) > 0 THEN 1 ELSE 0 END NumDays,
ClassId
FROM Table2
GROUP BY ClassId) as SUB
) AS a
PIVOT (SUM(TotDays) FROM ClassId IN ([12],[13],[14],[15])) AS PT
Is it possible to access a temporary column that was defined in a query for a Common Table Expression? Say I have
select * from myTable
;with cte as
(
select
*, Salary * 4 as FourTimesSalary
from
Employees
where
Name = #name
and ID >= 100
)
Is there a way to use the temporary column FourTimesSalary when querying cte like so?
select top 2 *
from cte
order by FourTimesSalary, Name
TIA.
Yes you can do that. Example:
with temp as
(
select 1 as id, 2*4 as val
UNION
select 2 as id, 3*4 as val
)
SELECT * FROM temp ORDER BY VAL desc
Your example looks fine, did you get an error when you tried that or something?
I'm trying to select records with a statement
SELECT *
FROM A
WHERE
LEFT(B, 5) IN
(SELECT * FROM
(SELECT LEFT(A.B,5), COUNT(DISTINCT A.C) c_count
FROM A
GROUP BY LEFT(B,5)
) p1
WHERE p1.c_count = 1
)
AND C IN
(SELECT * FROM
(SELECT A.C , COUNT(DISTINCT LEFT(A.B,5)) b_count
FROM A
GROUP BY C
) p2
WHERE p2.b_count = 1)
which takes a long time to run ~15 sec.
Is there a better way of writing this SQL?
If you would like to represent Set Difference (A-B) in SQL, here is solution for you.
Let's say you have two tables A and B, and you want to retrieve all records that exist only in A but not in B, where A and B have a relationship via an attribute named ID.
An efficient query for this is:
# (A-B)
SELECT DISTINCT A.* FROM (A LEFT OUTER JOIN B on A.ID=B.ID) WHERE B.ID IS NULL
-from Jayaram Timsina's blog.
You don't need to return data from the nested subqueries. I'm not sure this will make a difference withiut indexing but it's easier to read.
And EXISTS/JOIN is probably nicer IMHO then using IN
SELECT *
FROM
A
JOIN
(SELECT LEFT(B,5) AS b1
FROM A
GROUP BY LEFT(B,5)
HAVING COUNT(DISTINCT C) = 1
) t1 On LEFT(A.B, 5) = t1.b1
JOIN
(SELECT C AS C1
FROM A
GROUP BY C
HAVING COUNT(DISTINCT LEFT(B,5)) = 1
) t2 ON A.C = t2.c1
But you'll need a computed column as marc_s said at least
And 2 indexes: one on (computed, C) and another on (C, computed)
Well, not sure what you're really trying to do here - but obviously, that LEFT(B, 5) expression keeps popping up. Since you're using a function, you're giving up any chance to use an index.
What you could do in your SQL Server table is to create a computed, persisted column for that expression, and then put an index on that:
ALTER TABLE A
ADD LeftB5 AS LEFT(B, 5) PERSISTED
CREATE NONCLUSTERED INDEX IX_LeftB5 ON dbo.A(LeftB5)
Now use the new computed column LeftB5 instead of LEFT(B, 5) anywhere in your query - that should help to speed up certain lookups and GROUP BY operations.
Also - you have a GROUP BY C in there - is that column C indexed?
If you are looking for just set difference between table1 and table2,
the below query is simple that gives the rows that are in table1, but not in table2, such that both tables are instances of the same schema with column names as
columnone, columntwo, ...
with
col1 as (
select columnone from table2
),
col2 as (
select columntwo from table2
)
...
select * from table1
where (
columnone not in col1
and columntwo not in col2
...
);
What does adding ranked to a mysql query do?
I'm trying code from this post
SELECT * FROM ( SELECT #row := #row +1 AS rownum, [column name] FROM ( SELECT #row :=0) r, [table name] ) ranked WHERE rownum % [n] = 1
"ranked" is an alias that you're giving to your sub-select. You're just omitting the "AS" keyword, which is allowed in MySQL.
Example: SELECT name from MyTable AS table1
Nothing, apparently, as "ranked" isn't a MySQL keyword that I can find. Did you possibly mean "ordered by"?
I have a SQL Server 2008 query
SELECT TOP 10 *
FROM T
WHERE ...
ORDER BY ...
I'd like to get also the total number of the rows. The obious way is to make a second query
SELECT COUNT(*)
FROM T
WHERE ...
ORDER BY ...
Is there an efficient method?
Thanks
Do you want a second query?
SELECT TOP 10
*, foo.bar
FROM
T
CROSS JOIN
(SELECT COUNT(*) AS bar FROM T WHERE ...) foo
WHERE
...
ORDER BY
...
OR
DECLARE #bar int
SELECT #bar = COUNT(*) AS bar FROM T WHERE ...
SELECT TOP 10
*, #bar
FROM
T
CROSS JOIN
(SELECT COUNT(*) AS bar FROM T WHERE ...) foo
WHERE
...
ORDER BY
...
Or (Edit: using WITH)
WITH cTotal AS
(
SELECT COUNT(*) AS bar FROM T WHERE ...)
)
SELECT TOP 10
*, cTotal .bar
FROM
T
WHERE
...
ORDER BY
...
What is in this answer seems to work:
https://stackoverflow.com/a/19125458/16241
Basically you do a:
SELECT top 100 YourColumns, TotalCount = Count(*) Over()
From YourTable
Where SomeValue = 32
TotalCount will have the total number of rows. It is listed on each row though.
When I tested this the query plan showed the table only being hit once.
Remove the ORDER BY clause from the 2nd query as well.
No.
SQL Server doesn't keep COUNT(*) in metadata like MyISAM, it calculates it every time.
UPDATE: If you need an estimate, you can use statistics metadata:
SELECT rows
FROM dbo.sysindexes
WHERE name = #primary_key,
where #primary_key is your table's primary key name.
This will return the COUNT(*) from last statistics update.
SELECT TOP (2) *,
(SELECT COUNT(*) AS Expr1 FROM T) AS C
FROM T