I am trying to get a list of fruits eaten by different people.
+-----------+
ID Name
+----+------+
1 Paul
2 John
3 Nick
+----+------+
Table: users
+--------+--------+
userID Fruit
+--------+--------+
1 Apple
2 Peach
2 Orange
2 Apple
3 Apple
3 Peach
+--------+--------+
Table: eats
Now I can easily get a list of who ate what. But if I want to know who ate what fruit 2nd? Result would be:
+--------+----------+
Name Fruit
+--------+----------+
John Orange
Nick Peach
+--------+----------+
Result
SELECT Name, Fruit FROM users, eats WHERE ID = userID ... ???
As #lanzz points out, you can't count on the order. Maintaining a sequence would be expensive, but let's say you're storing the timestamp and the rows are returned in order...
Use a variable to remember how many rows you've seen for each name. Iterate the variable while looking at the same person, and reset it when you hit a new person. Then spit out only the records that have the desired sequence. (This has the added benefit of being able to select the user's nth row for any value of n).
set #seen_name = null;
set #seq = 1;
select * from (
SELECT Name, Fruit,
case when #seen_name is null OR #seen_name != name then #seq := 1 else #seq := #seq + 1 end as seq_formula,
#seen_name := name,
#seq as seq
FROM users, eats
WHERE ID = userID
) foo
where seq = 2;
and voila:
+------+--------+-------------+--------------------+------+
| Name | Fruit | seq_formula | #seen_name := name | seq |
+------+--------+-------------+--------------------+------+
| John | Orange | 2 | John | 2 |
| Nick | Peach | 2 | Nick | 2 |
+------+--------+-------------+--------------------+------+
This query ought to join the tables:
SELECT eats.Fruit, users.Name FROM eats, users WHERE users.ID = eats.userID
You cannot rely on row order in a relational database without ordering the rows explicitly, as the order of the returned rows will vary as you delete and insert new records in your table. You will need to have an additional column, say sequence, that you will explicitly store the order of the fruits consumed. Then, it is straightforward to select the fruit with sequence = 2.
Related
I have the following table:
Name Product
Bob Car
Bob Apples
Bob Pears
Bob Car
John Apples
John Pears
Whoever has bought a Product Car, I want to keep separate from everyone else. So, I create a flag:
Name Product Flag
Bob Car 1
Bob Apples 0
Bob Pears 0
Bob Car 1
John Apples 0
John Pears 0
But the problem with my flag is that even if I do a where condition and say, show me the consumer WHERE flag !=1, it'll pick Bob. Which is incorrect as Bob owns a car.
I would still like to GROUP by Product.
How do I separate the above table into two groups?
Thanks!
Use below query :-
select name from table where flag!=1
and name not in (select name from table where flag = 1)
group by name
"show me the consumer WHERE flag !=1, it'll pick Bob" that is because you are asking for rows where flag != 1. Instead you'll need something a little more complicated, like:
SELECT DISTINCT Name
FROM tableTable
WHERE Name NOT IN (SELECT Name FROM theTable WHERE Product = 'Car')
alternatively, you can do a LEFT JOIN, which may or may not be faster depending on the amount of data you have and how its values are distributed.
SELECT DISTINCT a.Name
FROM theTable a
LEFT JOIN theTable b ON a.Name = b.Name AND b.Product = 'Car'
WHERE a.Product != 'Car' AND b.Product IS NULL
;
This gets all the rows with products other than cars, and then uses the LEFT JOIN in conjunction with the IS NULL condition to find which did not also have a 'Car' row.
I think you want your table's data displayed, just with "People who bought cars" partitioned (not grouped) separately somehow - this could be done with an ORDER BY OwnsACar clause, for example.
Step 1: Identify the people who have bought cars:
SELECT DISTINCT
Name
FROM
yourTable
WHERE
Product = 'Car'
Step 2: Join on this data to generate a calculated "OwnsACar" column:
SELECT
yourTable.Name,
yourTable.Product,
ISNULL( carowners.Name ) AS OwnsACar
FROM
yourTable
LEFT OUTER JOIN
(
SELECT DISTINCT
Name
FROM
yourTable
WHERE
Product = 'Car'
) AS carowners ON carowners.Name = yourTable.Name
ORDER BY
OwnsACar ASC,
yourTable.Name ASC
You can use these two queries. The additional Flag column is not required.
-- do not have Car
SELECT *
FROM products
WHERE Name not in (SELECT DISTINCT Name
FROM products
WHERE Product='Car');
-- have Car
SELECT *
FROM products
WHERE Name in (SELECT DISTINCT Name
FROM products
WHERE Product='Car');
Illustration:
-- table
SELECT * FROM products;
+------+---------+
| Name | Product |
+------+---------+
| Bob | Car |
| Bob | Apples |
| Bob | Pears |
| Bob | Car |
| John | Apples |
| John | Pears |
+------+---------+
-- query for people that do not have Car
+------+---------+
| Name | Product |
+------+---------+
| John | Apples |
| John | Pears |
+------+---------+
-- query for people having 'Car'
+------+---------+
| Name | Product |
+------+---------+
| Bob | Car |
| Bob | Apples |
| Bob | Pears |
| Bob | Car |
+------+---------+
Try with :
SELECT `t`.`Name`, `t`.`Product`, SUM(`t`.`Flag`) as hasCar
FROM your_table t
GROUP BY `t`.`Name`
HAVING `t`.`hasCar` = 0;
Although you can go without the flag column by going :
SELECT `t`.`Name`, `t`.`Product`, SUM(IF(`t`.`Product` = 'Car', 1, 0)) as hasCar
FROM your_table t
GROUP BY `t`.`Name`
HAVING `t`.`hasCar` = 0;
Sorry if my question sounds misleading, I'm not fully sure how to formulate it.
Consider the following tables: Fruit which has an ID and Name, Person which also has an ID and Name, and Person_has_Fruit (many to many linking table) which has a Person_ID and a Fruit_ID.
What transaction can retrieve the people which have two or more specific fruits? Basically how can I intersect the results?
Example:
Fruit table
ID | Name
-----------------
1 | Apple
2 | Pineapple
3 | Banana
4 | Lemon
Person table
ID | Name
-----------------
1 | Tom
2 | Bill
3 | John
Many to many table
PersonID | FruitID
-----------------
3 | 1
1 | 2
3 | 2
2 | 3
3 | 3
I want a query to retrieve me John when I ask for the person which has Apple, Pineapple and Banana. Any suggestions?
I've tried 'SELECT * FROM Person_has_Fruit WHERE FruitID in ('1', '2', '3')' but that is incorrect as it retrieves all the person IDs which have one of them, so basically it implies an or relationship between the values.
You are looking to perform a relational division between the some tuples of Fruit table and the Person table.
So:
Select * From Person p
Where Not Exists ( Select * from Fruit f
Where (Name = 'Apple' Or Name = 'Pinneapple' or Name = 'Banana')
And Not Exists ( Select * from Person_has_Fruit pf
Where pf.PersonId = p.ID and pf.FruitId=f.ID))
Try create a view first.
CREATE VIEW fruitowners AS
( SELECT personfruit.personID, fruittable.id, persontable.person, fruittable.fruit
FROM persontable, fruittable, personfruit
WHERE personfruit.personID=persontable.id AND personfruit.fruitID=fruittable.id )
Then:
SELECT fruit FROM fruitowners
WHERE person='John'
returns all the fruit types John owns
And:
SELECT person FROM fruitowners
WHERE fruit='Banana'
returns all the banana owners.
And:
SELECT COUNT(*) FROM fruitowners WHERE person='John'
returns the number of fruits John owns
And:
SELECT COUNT('fruit') FROM fruitowners WHERE person='John' and fruit='Banana'
returns how many bananas John owns
For example I have a table with three fields:
id (int)
name (varchar)
company (int)
Let's say that I have the following data (example only)
id --- name --- company
---------------------------------------
1 --- John Baker --- 1
2 --- Ann Johnson --- 1
3 --- John Wu --- 1
4 --- Mike Johns --- 2
5 --- John John --- 2
6 --- Johnny Boy --- 2
I would like perform a search on name, and return the data staggered by company. So if I perform a search on LIKE '%John%' , I wish to return the data in a way where it is sorted by company like: 1, 2, 1, 2, 1, 2 whilst maintaining as much relevancy in return order to the original search term as possible.
I have no idea how to return the data in this staggered way, and I have thought about it for hours. If somebody can please help me I'd love to hear their ideas!
This would be rather easy if we could use SQL standard functions to ROW_NUMBER each name PARTITIONed BY company. (ROW_NUMBER gives us a "naive ranking" or simple numbering, with no ties and no gaps.) This is essentially #zebediah49's proposal in his comment. MySQL, sadly, cannot today do this.
#GordonLindoff's answer simulates this functionality in MySQL with a self-join technique. Here's another way to do the same.
First we group every person by company and then globally naively rank them, using a user variable. So, the three people in company A become #1, #2, and #3, and the two people in company B become #4 and #5, and so on:
company | name == ROW_NUMBER()d after ==> company | name | rank
--------+-------- == GROUP BY `company` ==> --------+---------+------
A | Alice A | Alice | 1
B | Bob A | Charlie | 2
A | Charlie A | Deborah | 3
A | Deborah B | Bob | 4
B | Erwin B | Erwin | 5
The user variable technique to simulate ROW_NUMBER in MySQL is easy to search for, but here's a compact demonstration from another SO answer.
Now if we MOD a global naive rank by the number of people within a company, we get a "partitioned rank", a relative rank within the company:
company | name | rank | npeople | rank % npeople
--------+----------+------+---------+----------------
A | Alice | 1 | 3 | 1
A | Charlie | 2 | 3 | 2
A | Deborah | 3 | 3 | 0
B | Bob | 4 | 2 | 0
B | Erwin | 5 | 2 | 1
Putting it all together, JOINing against a query to count the number of people in each company, we get:
SELECT id, name, ranked.company
FROM ( SELECT tbl.id, tbl.name, tbl.company, (#rn := #rn + 1) AS rn
FROM tbl
JOIN (SELECT #rn := 0) vars
WHERE tbl.name LIKE '%John%'
ORDER BY company) ranked
JOIN (SELECT company, COUNT(id) AS npeople FROM tbl GROUP BY company) companies
ON ranked.company = companies.company
ORDER BY rn MOD companies.npeople, company
If you want it sorted by company:
select *
from t
where . . .
order by company, id;
If you want it interleaved, then a counter within a company helps. Here is one way:
select t.*
from (select *,
(select count(*) from t t2 where <where clause on t2 here> and t2.comapny = t.company and t2.id < t.id) as seqnum
from t
where . . .
) t
order by seqnum, company
One possible solution:
select *
from yourTable
where ...
order by ((company*1000) + id);
Add as much zeros as you need. At least, you'll need as much zeros as this number:
select pow(10,length(max(company))) from yourTable;
The ordering may be quite slow if you pull a lot of records in this query, so I suggest you use an optimal where condition.
Not the most elegant solution, but it may work.
I have a table like:
Name | ID | Event
Smith| 1 |
Smith| 2 | Y
Smith| 3 |
Jones| 1 |
Jones| 2 | Y
Jones| 3 |
Jones| 4 | Y
I'd like to count the number of times an Event has been seen for each person at each point, e.g.:
Name | ID | Event | Event Count
Smith| 1 | | 0
Smith| 2 | Y | 1
Smith| 3 | | 1
Jones| 1 | | 0
Jones| 2 | Y | 1
Jones| 3 | | 1
Jones| 4 | Y | 2
I'm guessing I can't do this in SQL? If not, can you be very clear how I go about doing this in SAS (or whatever way is appropriate), as I am new to this!
(FYI, this is leading to me being able to differentiate rows that happen before or after each event - i.e. filter by Event = blank, and anything 0 happened before the first event, anything 1 after, etc. There might be an easier way to do this.)
Thanks!
If you want to go down the SAS route, it reads data sequentially so is very good at this type of problem
data have;
infile datalines missover;
input Name $ ID Event $;
datalines;
Smith 1
Smith 2 Y
Smith 3
Jones 1
Jones 2 Y
Jones 3
Jones 4 Y
;
run;
proc sort data=have;
by name id;
run;
data want;
set have;
by name id;
if first.name then event_count=0;
event_count+(event='Y');
run;
You could potentially do something like this in a query:
select Name, ID, Event,
(
select count(*)
from MyTable
where Name = t.Name
and Event = 'Y'
and ID <= t.ID
) as EventCount
from MyTable t
The correlated subquery will find this count for you, though this is something of a triangular join (SQL Server link, but still applicable), so performance isn't wonderful.
Here is the SQL Fiddle showing the result.
Note that this should work in virtually any RDBMS.
SELECT Name, ID, Event, grpTotal
FROM
(
select Name,
ID,
Event,
#sum := if(#grp = Name,#sum,0) + if(`Event` = 'Y',1,0) as grpTotal,
#grp := Name
from TableName,
(select #grp := '', #sum := 0) vars
order by Name, ID
) s
SQLFiddle Demo
I've got a table containing persons gender-coded as 0 and 1. I need to select every other row as male/female. I thought I could manage this somehow by using modulo and the gender-codes 0 and 1, but I haven't managed to figure it out yet...
The result I'm looking for would look like this:
+-----+--------+-------+
| row | gender | name |
+-----+--------+-------+
| 1 | female | Lisa |
| 2 | male | Greg |
| 3 | female | Mary |
| 4 | male | John |
| 5 | female | Jenny |
+-----+--------+-------+
etc.
The alternative is to do it in PHP by merging 2 separate arrays, but I would really like it as a SQL query...
Any suggestions are appreciated!
Do two subqueries to select male and female. Use ranking function to have them enumerated.
Males:
1 | Peter
2 | John
3 | Chris
Females:
1 | Marry
2 | Christina
3 | Kate
Then multiplay ranking result by x10 and add 5 for females. So you have this:
Males:
10 | Peter
20 | John
30 | Chris
Females:
15 | Marry
25 | Christina
35 | Kate
Then do the UNION ALL and sort by new sort order/new ID.
Together it should like this (pseudo code)
SELECT
Name
FROM
(subquery for Males: RANK() AS sortOrd, Name)
UNION ALL
(subquery for Females: RANK()+1 AS SortOrd, Name)
ORDER BY SortOrd
Result should be like this:
Males and Females:
10 | Peter
15 | Marry
20 | John
25 | Christina
30 | Chris
35 | Kate
Found Emulate Row_Number() and modified a bit for your case.
set #rownum := 0;
set #pg := -1;
select p.name,
p.gender
from
(
select name,
gender,
#rownum := if(#pg = gender, #rownum+1, 1) as rn,
#pg := gender as pg
from persons
order by gender
) as p
order by p.rn, p.gender
Try on SQL Fiddle
Note: From 9.4. User-Defined Variables
As a general rule, you should never assign a value to a user variable
and read the value within the same statement. You might get the
results you expect, but this is not guaranteed.
I will leave it up to you do decide if you can use this. I don't use MySQL so I can't really tell you if you should be concerned or not.
Similar to Mikael's solution but without the need to order the resultset multiple times -
SELECT *
FROM (
SELECT people.*,
IF(gender=0, #mr:=#mr+1, #fr:=#fr+1) AS rank
FROM people, (SELECT #mr:=0, #fr:=0) initvars
) tmp
ORDER BY rank ASC, gender ASC;
To avoid having to order both the inner and outer selects I have used separate counters (#mr - male rank, #fr - female rank) in the inner select.
I've got a table containing persons gender-coded as 0 and 1
Then why would you make assumptions on the order of rows in the result set? Seems to me transforming the 0/1 into 'male'/'female' is far more robust:
select name, case gender when 0 then 'male' else 'female' end
from Person
SELECT alias.*, ROW_NUMBER() OVER (PARTITION BY GENDER ORDER BY GENDER) rnk
FROM TABLE_NAME
ORDER BY rnk, GENDER DESC