I have a column in my table called student_id, and I am storing the student IDs associated with a particular record in that column, delimited with a | character. Here are a couple sample entries of the data in that column:
243|244|245
245|1013|289|1012
549|1097|1098|245|1099
I need to write a SQL query that will return records that have a student_id of `245. Any help will be greatly appreciated.
Don't store multiple values in the student_id field, as having exactly one value for each row and column intersection is a requirement of First Normal Form. This is a Good Thing for many reasons, but an obvious one is that it resolves having to deal with cases like having a student_id of "1245".
Instead, it would be much better to have a separate table for storing the student IDs associated with the records in this table. For example (you'd want to add proper constraints to this table definition as well),
CREATE TABLE mytable_student_id (
mytable_id INTEGER,
student_id INTEGER
);
And then you could query using a join:
SELECT * FROM mytable JOIN mytable_student_id
ON (mytable.id=mytable_student_id.mytable_id) WHERE mytable_student_id.student_id = 245
Note that since you didn't post any schema details regarding your original table other than that it contains a student_id field, I'm calling it mytable for the purpose of this example (and assuming it has a primary key field called id -- having a primary key is another requirement of 1NF).
#Donut is totally right about First Normal Form: if you have a one-to-many relation you should use a separate table, other solutions lead to ad-hoccery and unmaintainable code.
But if you're faced with data that are in fact stored like that, one common way of doing it is this:
WHERE CONCAT('|',student_id,'|') LIKE '%|245|%'
Again, I agree with Donut, but this is the proper query to use if you can't do anything about the data for now.
WHERE student_id like '%|245|%' or student_id like '%|245' or student_id like '245|%'
This takes care of 245 being at the start, middle or end of the string. But if you aren't stuck with this design, please, please do what Donut recommends.
Related
Let's say there is a database with two tables: one customer table and one country table. Each customer row contains (among other things) a countryId foreign key. Let's also assume that we are populating the database from a data file (i.e., it is not an operator that is selecting a country from a UI).
What is the best practice for this?
Should one query the database first and get all ID's for all countries, and then just supply the (now known) country id's in the insert query? This is not a problem for my 'country' example, but what if there is a large number of records in the table that is being referred?
Or should the insert query use a sub query to get the country id based on the country name? If so, what if the record for the country does not exist yet and has to be added?
Or another approach? Or does it depend? :)
I would suggest using a join in your insert query to get the country id based on the country name. However, I don't know if that's something possible with every SGBD and you don't give more precision on the one you're using.
I know it's possible to have n amount of columns, but is it proper mysql "coding standard"?
Here is what I'm doing:
I am a table student which includes all the students info including testScores:
student
-------
studId
name
age
gender
testId
Instead of putting each individual test answer within the student table, I made a separate table called testAnswers that will hold each students test results:
testAnswers
-----------
testId
ques1
ques2
.
.
.
quesN
Each entry in the testAnswers table corresponds to a specific student in the table student.
Of course, there will be an admin that will be able to add questions and remove questions as each year the test questions may change. So, if the admin were to remove an answer, than that means one of the columns would be removed.
Just to reiterate myself, I know this is possible to edit and remove columns in a table in mysql, but is good "coding standard"?
The answer is a simple and clear: No. That's just not how you should do it except for very few corner cases.
The usual way to approach this is to normalize your database. Normalization follows a standard procedure that (among other things) avoids having a table with columns names ques1, ques2, ques3 ....
This process will lead you to a database with three tables:
students - id, name, and other stuff that applies to one student each
questions - id and question text for each question
answers - this is a N:M relation between students: student_id, question_id, answer_value
Use two tables!
What you are describing is a one to many relationship as there can be one student to many test scores. You would need to have some id as a foreign key to the student_id and put this id in the testAnswers table. You can then set constraints, which tell the database how to handle removal of data.
As one commenter has mentioned, using one table would result in breaking 1nf or first normal form which basically says that you cannot have multiple values for a single column given a particular record - You can't have multiple test scores for the same user in a given table, instead break the data up into two tables.
...of course 2 tables, also could use 3, just remember to insert a studId column also in the testAnswers table (with REFERENCE to the student table) and an INNER JOIN testAnswers ON student.studId=testAnswers.studId at the SELECT query (to read the data).
Disclaimer: I didn't design these tables, and am aware they're not the best way to store this data. I need advice on the best way to handle the situation.
I have two MySQL tables:
`students`
id int; primary, unique
prof1_id varchar
prof2_id varchar
prof3_id varchar
`professors`
id int; primary, unique, auto-increment
username varchar
fname varchar
fname varchar
students_id int
comment text
A professor's username may be in any of the last three columns in the students table. Professors will need to provide one comment for each student who has them in their row in the students table.
The application that is the front end to this database expects two more columns in the professors table: student_id and comment. Since each student may have any 3 professors in their row, new rows will need to be added to professors whenever they are listed for multiple students. This means that the professors id field will need to auto increment for each new student comment.
What is the best way to accomplish this data transfer with a MySQL query? I've tried looking at UPDATE in combination with JOIN, but I'm not sure there is even a valid way to do this. Also, do I need to create another primary key for professors since the multiple rows will have the same id?
I suspect that a VIEW might be the best way to accomplish this, but the application on the front end expects the information to be stored in the professors table.
One other thing I was considering was that I could create a new table by joining the two tables and have a two-column primary key, students.id, professor.id.
Thanks!
Also, do I need to create another primary key for professors since the
multiple rows will have the same id?
Yes. This is good idea.
I wrote simple query that merges data from first table into 2 columns. This is not complete answer, but it can help you a lot:
SELECT id, prof1id as profid
UNION
SELECT id, prof2id
UNION
SELECT id, prof3id
UNION;
You may use this for view, inserts, but im not familiar with specyfic MySQL syntax and i dont want to misslead you. Please give feedback if it work.
"UNION" removes duplicate rows, you may need to use "UNION ALL" to keep duplicates (like duplicated values in 2 or 3 professors columns).
I'm wondering how to store a list of ID indexes for each id in a table. These will effectively form "relations" between the IDs. Here's the main table so far:
table main:
id, integer, primary key
name, varchar
text_info_1, varchar
text_info_2, varchar
And now for each row, there will be some list of other rows' IDs which will show me how one row relates to the next. For example, the row with id #5 might be related to IDs 6,7,9,25,...etc.
Here are the options I've considered:
Create a new column as a text field and store a serialized list of these integer values. Then just unserialize when I want them.
Create a new table called "relations" with columns relation_id (int auto increment primary key), name1, name2, [and optional other fields specifying the relation type, which would be nice].
I feel like option 1 is a bit of a hack. I've done it before and it works, but perhaps option 2 is better design?
I'm worried about speed though. With option 1 I can just do SELECT relations FROM main WHERE id = $id, and then unserialize the result and have my array with integer indices. But with option 2 I'll have to browse through a table that will be many times (10x or more) larger, and do "SELECT name1, name2 FROM relations".
Speed is my main priority here. I'm not sure which one is better for space, though I would be curious to find out.
So which option should I go with? Are there other good options I haven't considered? I'd also appreciate some good general pointers on database design!
Thanks a bunch.
Second option is better. If you want to get better in database design, you should read about database normalization. Sadly, all the materials I've used were not in English. This wikipedia link
could be a start.
Table relations:
relation_id (int auto increment primary key)
id_one (int)
id_two (int)
...
Instead of storing names in the relations table, you store ids.
my website includes a mysql db with two tables.
one table has a field called 'id_array', and it contains a string look like this:
'1,3,7,78,89,102'. it represents an array of id's in the second table.
what is the best strategy for retrieving the id's in the second table?
i want the fastest way.
currently i am using 'SELECT * FROM first_table WHERE id IN (id_array)'.
i used to query one by one, but i assume it is a lot slower.
if someone has a better structure to offer me for the db, that would be great.
this structure is the best i came up with, but i'm pretty sure it is not so efficiant.
i need a fast and convenient way to find all the id's that belong to the id from the first table.
help would be appreciated.
A comma separated value means the data is denormalized -- the IN clause won't work for you, because it works on distinct values.
Short Term Solution
Use MySQL's FIND_IN_SET function, like this:
SELECT *
FROM first_table
WHERE FIND_IN_SET(id, id_array)
Long Term Solution
...is not to store values in this manner, which means having a table to store the distinct values and a table to join that table of distinct values to the original table. IE:
FIRST_TABLE
first_table_id (primary key)
FIRST_TABLE_TYPE_CODES
type_code_id (primary key)
FIRST_TABLE_TYPE_MAP
first_table_id (primary key)
type_code_id (primary key)
You have a field that holds the ids that the record is related to in another table? You might want to create a lookup table that links the two tables together to help you with a many to many relationship. Post a little bit more or your db setup