I created three tables "student" and "course" and "lecturer" and I inserted data into them.
Now I want to retrieve some data by select.
When I want to show: Subject taken by Kumar
SELECT STUDENT.NAME, COURSE.SUBJECT1, COURSE.SUBJECT2, COURSE.SUBJECT3
FROM STUDENT,COURSE
WHERE STUDENT.COURSE = COURSE.COURSE = 'MLVK'
it repeats the data more than one time.
I hope anyone help me
All the best
Learn to use proper, explicit, standard JOIN syntax.
And, table aliases:
SELECT s.NAME, c.SUBJECT1, c.SUBJECT2, c.SUBJECT3
FROM STUDENT s JOIN
COURSE c
ON s.COURSE = c.COURSE
WHERE c.COURSE = 'MLVK'
If I were going to use these tables to pick subjects taken by Kumar, I would write something like:
SELECT
s.name, c.course, c.subject1, c.subject2, c.subject3
FROM
student as s
LEFT JOIN course as c on c.course = s.course
WHERE
s.no_matrik = '23456'
GROUP BY
s.name, c.course, c.subject1, c.subject2, c.subject3
I think this version makes the intent slightly more clear (pick subject for a particular student, Kumar) than the previous answer (select subjects for any student having course = 'MLVK'). This answer will also return information for Kumar even if he has no course value in the Student table (pre-enrollment?).
First of all I would suggest that to model your data properly in BCNF form, where you should have modelled another table to persist subjects and map the lecturer who take that subject.
Ex: (Subject Table)
SubjectId LECT_ID
---------- ------------
TT234 L123
TT235 L003
and, your Course table would be more of course to subject mapping, like:
CourseName SubjectId
------------- --------------
DTM TT235
DTM TT695
...
then you use query as:
Select sub.SubjectId, l.NAME
From
Student s JOIN
Course c
on c.CourseName = s.COURSE
JOIN Subject sub
on sub.SubjectId = c.SubjectId
JOIN Lecturer l
on l.LECT_ID = sub.Lecturer
Where s.NAME = 'Aminah'
the above query will result as:
SubjectId NAME
--------- ----------
PP563 Ahmad
SS003 Ahmad
PP999 John
as Ahmad happens to be teaching 2 subjects in course DPG. but if you wish to report distinct Lecturers for Aminah, you can change query as:
SELECT NAME FROM Lecturer where LECT_ID
in (
Select l.LET_ID
From
Student s JOIN
Course c
on c.CourseName = s.COURSE
JOIN Subject sub
on sub.SubjectId = c.SubjectId
JOIN Lecturer l
on l.LECT_ID = sub.LECT_ID
Where s.NAME = 'Aminah'
) a
What currently happens is it just selects students from CA and students who like to skateboard. I need it to return only students who are both from CA and play soccer.
SELECT *
FROM schooldata a
INNER JOIN studentinfo b
ON b.schooldata_id = a.id
WHERE a.state = "ca"
AND ( activity = "soccer"
OR activity = "skateboard" )
You will have to do inner join based on lastname and firstname column ex:-b.lastname=a.lastname and b.firstname=a.firstname . ideally you should be maintaining primary key column of type integer in schooldata table and its foreign key reference in studentinfo and join based on those columns.
You should use join clauses. And I think base on your question. Inner Join is the best clause you should use.
SELECT column_name(s)
FROM table1
INNER JOIN table2 ON table1.column_name = table2.column_name;
I'm fairly new to MySQL, and trying to understand the many-to-many relationship since these examples can popup in interviews
There are 3 tables, and since a Student can have many courses and a Course can have many students, this is a Many-to-Many relationship right?
The tables are
Student- has student ID, name, date of birth, and department.
Courses- Has ID, Name of course
Student_Courses- Has student_id, course_id
How would I display these 2 questions-
1) Given a studentID, return all the names of the courses the student is taking
2) Return the name of students who is taking X amount of courses or more (Ex. 4 or more courses).
Im trying to write queries on these, but I'm stuck...
In the case of selecting all of the courses for a given student ID you could try the following, which will return one row for each Course a Student is associated with.
select
s.name as StudentName,
c.name as CourseName
from `Student` as s
inner join `Student_Course` as sc on (sc.student_id = s.ID)
inner join `Course` as c on (c.ID = sc.course_id)
where
(s.`ID` = 'given_Student_ID_here')
;
As for selecting a list of the names of Students taking N or more courses, for this you might use an aggregating sub-select as a WHERE clause in which we reference one of the outer tables (i.e. [Student]) so that the result of the aggregation is personalised per Student record:
select
s.name as StudentName
from `Student` as s
where
(
(
select count(*)
from `Student_Course` as sc
inner join `Course` as c on (c.ID = sc.course_id)
where (sc.student_id = s.ID)
) >= 4
)
;
You might also consider an alternative approach to this second problem by using the GROUP BY and HAVING clauses:
select
s.name as StudentName
from `Student` as s
inner join `Student_Course` as sc on (sc.student_id = s.ID)
inner join `Course` as c on (c.ID = sc.course_id)
group by
s.name
having
count(*) >= 4
;
I have four schemas:
takes(ID,course_id,sec_id,semester,year)
student(ID,name,dept_name,tot_credit)
course(course_id,title,dept_name,credits)
department(dept_name,building,budget)
I want to create a query that finds the name and id of each Astronomy student whose name begins with the letter âTâ and who has not taken at least 16 Astronomy courses.
What's the easiest way I could do this?
I already wrote this beginning bit
SELECT name, id
FROM student
WHERE dept_name='Astronomy' AND name LIKE '%T%'
I'm not quite sure how to finish this off.
Any help would be greatly appreciated :)
Here's the result
NAME ID CLASS_TAKEN
-------------------- ----- -----------
Tolle 38279 12
Teo 62268 13
Tolle 93223 13
Tsukamoto 17707 5
Titi 11576 9
Teo 91772 12
Toraichi 50387 11
Tewari 80754 14
Tiroz 64091 14
9 rows selected
I need Teo with the id 91772 and Tewari 80754 to be gone
Given my reading of the requirements and the comments it's pretty clear that the question is not very clear. :-) What you're looking for are students where (total # of courses given by the Astronomy department) - (# of Astronomy courses taken by student) >= 16. So, how do we find these values? First, let's start with the total number of courses given by the Astronomy department. This is pretty simple:
SELECT COUNT(*) AS ASTRONOMY_COURSE_COUNT
FROM COURSE
WHERE DEPT_NAME = 'ASTRONOMY'
Now, the second part is to determine how many courses given by the Astronomy department each student has taken. To do this we need to start with the student, join to the courses the student has taken (the TAKES table), then join to the COURSES table to find out which department each course is part of. Something like the following should do it:
SELECT s.ID, s.NAME, COUNT(*) AS STUDENT_ASTRO_COUNT
FROM STUDENT s
INNER JOIN TAKES t
ON t.ID = s.ID
INNER JOIN COURSE c
ON c.COURSE_ID = t.COURSE_ID
WHERE c.DEPT_NAME = 'ASTRONOMY' AND
s.NAME LIKE 'T%'
GROUP BY s.ID, s.NAME;
OK, now we need to put this together. You've tagged this question for both Oracle and MySQL so I'm going to guess you'll accept valid syntax for either database; thus I'll use Oracle Common Table Expression syntax to pull everything together:
WITH ASTRONOMY_COURSES AS (SELECT COUNT(*) AS ASTRONOMY_COURSE_COUNT
FROM COURSE
WHERE DEPT_NAME = 'ASTRONOMY'),
STUDENT_ASTRO_COURSES AS (SELECT s.ID,
s.NAME,
COUNT(*) AS STUDENT_ASTRO_COUNT
FROM STUDENT s
INNER JOIN TAKES t
ON t.ID = s.ID
INNER JOIN COURSE c
ON c.COURSE_ID = t.COURSE_ID
WHERE c.DEPT_NAME = 'ASTRONOMY' AND
s.NAME LIKE 'T%'
GROUP BY ID)
SELECT s.ID,
s.NAME,
s.STUDENT_ASTRO_COUNT,
a.ASTRONOMY_COURSE_COUNT - s.STUDENT_ASTRO_COUNT AS UNTAKEN_COUNT
FROM STUDENT_ASTRO_COURSES s
CROSS JOIN ASTRONOMY_COURSES a
WHERE a.ASTRONOMY_COURSE_COUNT - s.STUDENT_ASTRO_COUNT >= 16;
Note here that a CROSS JOIN is used to put together the subqueries. This means that all the rows of each subquery are joined to all the rows of the other subquery - but since in this case the ASTRONOMY_COURSES subquery will only return a single row what we're doing is appending the ASTRONOMY_COURSE_COUNT value onto each row returned by the STUDENT_ASTRO_COURSES subquery.
That should at least get you pretty close. Amend as needed.
Not tested on animals - you'll be first! :-)
Share and enjoy.
Do you need to use all tables?
Table department has no links with the student,
Table takes has no links with the student,
Table coursehas no links with the student.
If student lists total credits that are all Astronomy I think this can be used:
select name, id, MAX(tot_credit) as credits
from student
where dept_name='Astronomy' and name like 'T%'
group by name, id
having MAX(tot_credit)<=16
PS - you schema is not good; PK-FK references are missing
Your query will need to reference more tables than just the student table.
Your tables seem be missing some important information, which student has taken which course. There's a table named takes, but there doesn't appear to be any relationship between takes and student.
So first, figure out how to list the students along with the Astronomy courses they have taken. Each row will identify the student and a course.
SELECT s.id AS student_id
, s.name AS student_name
, t.???
FROM student s
JOIN ??? t
ON t.student_id = s.id
WHERE ...
You may also need to include another "join" to an additional table, in order to identify which course a student has taken is an Astronomy course.
To also include students that have not take any Astronomy courses, you can use an outer join, rather than an inner join. (That would mean including the LEFT keyword before JOIN, and relocating predicates from the WHERE clause to the ON clause. (A predicate in the WHERE clause that can only be satisfied by non-NULL values will negate the outer-ness of the join.)
Once you have a query that returns that set (students along with any astronomy courses they've taken), you can then add a GROUP BY clause to "collapse" a set of rows into a single row. (Looks like you want the rows "grouped" by student.)
And then an aggregate function like COUNT() or SUM() can be used to get a count of rows for each group. (If you don't want to count any re-takes of a course (a "duplicate" course for a student) you may be able to make use of the COUNT(DISTINCT t.foo) form.
And then a HAVING clause can be added to the query, to compare the value returned from the aggregate expression to a constant value, to return only rows that satisfy a specific condition.
FOLLOWUP
Assuming:
CREATE TABLE course
( id INT UNSIGNED NOT NULL COMMENT 'PK'
, title VARCHAR(30) NOT NULL COMMENT 'course title'
, dept_name VARCHAR(30) NOT NULL COMMENT 'FK ref dept.name'
, credits DECIMAL(5,2) COMMENT 'credit hours'
, PRIMARY KEY (id)
);
CREATE TABLE student
( id INT UNSIGNED NOT NULL COMMENT 'PK'
, name VARCHAR(30) NOT NULL COMMENT 'student name'
, dept_name VARCHAR(30) NOT NULL COMMENT 'FK ref dept.name'
, tot_credit INT COMMENT '?'
, PRIMARY KEY (id)
);
CREATE TABLE takes
( student_id INT UNSIGNED NOT NULL COMMENT 'FK ref student.id'
, course_id INT UNSIGNED NOT NULL COMMENT 'FK ref course.id'
, sec_id INT UNSIGNED NOT NULL COMMENT '?'
, semester INT UNSIGNED NOT NULL COMMENT '?'
, year INT UNSIGNED NOT NULL COMMENT '?'
, PRIMARY KEY (student_id, course_id, sec_id, semester, year)
, CONSTRAINT FK_takes_course FOREIGN KEY (course_id) REFERENCES course (id)
, CONSTRAINT FK_takes_student FOREIGN KEY (student_id) REFERENCES student (id)
);
Query to get a list of students...
SELECT s.id
, s.name
FROM student s
WHERE s.name LIKE 'T%'
AND s.dept_name = 'ASTRONOMY'
Get list of students along with the courses they've taken, returning the id of the ASTRONOMY courses they've taken...
SELECT s.id AS student_id
, s.name AS student_name
, c.id AS course_id
FROM student s
LEFT
JOIN takes t
ON t.student_id = t.id
LEFT
JOIN course c
ON c.id = t.course_id
AND c.dept_name = 'ASTRONOMY'
WHERE s.name LIKE 'T%'
AND s.dept_name = 'ASTRONOMY'
Collapse the rows to one per student using a GROUP BY, and use aggregate functions to get counts or totals.
SELECT s.id AS student_id
, s.name AS student_name
, SUM(c.credits) AS total_astronomy_credits_taken
, COUNT(c.id) AS count_astronomy_courses_taken
, COUNT(DISTINCT c.id) AS count_distinct_astronomy_courses_taken
FROM student s
LEFT
JOIN takes t
ON t.student_id = t.id
LEFT
JOIN course c
ON c.id = t.course_id
AND c.dept_name = 'ASTRONOMY'
WHERE s.name LIKE 'T%'
AND s.dept_name = 'ASTRONOMY'
GROUP
BY s.id
, s.name
To omit rows from this resultset, add a HAVING clause. For example, to exclude rows where total_astronomy_credits_taken is greater than or equal to 12...
HAVING total_astronomy_credits_taken >= 12
If you want the rows returned in a certain sequence, specify that in an ORDER BY clause
ORDER BY s.id
If you want to replace NULL values from the aggregates with zeroes, you can warp the aggregate expression in an IFNULL(foo,0) function, e.g.
, IFNULL(COUNT(c.id),0) AS count_astronomy_courses_taken
Try this :
select a.name, a.id, count(b.ID) as class_taken
from student a inner join takes b
on a.ID = b.ID
inner join course c
on b.course_id = c.course_id
where a.dept_name='Astronomy' and substring(a.name,1,1) = 'T'
group by a.name, a.id
having count(b.ID) < 17
ok, so there are 3 elements from the database which i am using for this query.
Person, with person_id as primary key,
Learners_to_classes, with has its own primary key and two foreign keys, class_id and person_id
and classes which has a primary key class_id.
Learners_to_classes has many to many relationships with person and classes, how ever the two foreign keys together must be unique (a person cant attend the same class twice.
each class has its own course_id (to define what type of class it is).
There is over 40 thousand records in learners_to_classes,
over 36 thousand records in person,
and around 1,300 records in classes.
What I am trying to do, is count the people who have done one type of course, but not the others.
SELECT count(distinct ltc.person_id) AS participants,
classes.classcode as classcode,
DATE_FORMAT(classes.course_start_date, '%Y') AS YEARPeriod
FROM learners_to_classes AS ltc
INNER JOIN classes ON ltc.class_id = classes.class_id
WHERE classes.deleted=0
AND classes.course_id=1
AND NOT EXISTS (SELECT *
FROM learners_to_classes AS o
INNER JOIN classes AS c ON o.class_id=c.class_id
WHERE (c.course_id=2 OR c.course_id=4)
AND o.person_id=ltc.person_id)
GROUP BY YEAR(classes.course_start_date), classcode
This query takes one hell of a long time too run, and i am hoping there might be a more efficient way to get the information.
Presumably, you want people who have taken course 1 but not course 4. You can do this with an aggregation query and a having clause. The following query returns the learners who have taken one class but not the other:
SELECT ltc.person_id
FROM learners_to_classes AS ltc INNER JOIN
classes c
ON ltc.class_id = c.class_id
WHERE c.deleted = 0
GROUP BY ltc.person_id
HAVING sum(c.course_id = 1) = 1 and
sum(c.course_id = 4) = 0;
If you want just the number of such people, use this as a subquery:
SELECT COUNT(*)
FROM (SELECT ltc.person_id
FROM learners_to_classes AS ltc INNER JOIN
classes c
ON ltc.class_id = c.class_id
WHERE c.deleted = 0
GROUP BY ltc.person_id
HAVING sum(c.course_id = 1) = 1 and
sum(c.course_id = 4) = 0
) l;