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.
Related
Let's say I have the below schema. country -> states -> district -> village.
I understand in the real scenario there won't be an auto-increment id for country states etc.
But let's assume that we have the below scenario.
I am using MySQL as a database.
these all tables are dependent on the previous table id.
for example, once the countryId is generated that would be used in state table, once stateId is generated that would be used in the district table and so on.
That's the real problem. In a crude way, I can add country object and insert in country table. Get the auto-generated country id and insert the record in state table and so on.
If I have 100 countries to insert then there will be billions of records to be inserted in the village table and the crude way is going to be a terrible way to do it.
I am looking for the fastest way to do it.
I am open for spring boot, JPA, and some other way also.
Any link, suggestion, idea is welocme.
I have a database named Football with a table named Club. My team columns are club_name, location, manager and established.
I need to know, how to add two or more names to same column. Like for example, in order to find location, I can type city instead of location. Or write coach instead of manager.
Or should I have to make another column with same data but different name.
Its not a good idea to have duplicate columns in the table, rather you can use column alias while retrieving the data. e.g.
In order to find location, and want its column name in the result like city you can write query :
SELECT location AS city FROM table_name;
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.
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
for example,access example db, every student record have a nested table about guardians.
http://img101.imageshack.us/img101/9881/53882937.jpg
No. Something like that is almost always done as a single mapping table for all students, with a foreign key column pointing to the student table to specify which student a particular row relates to.
You then just filter the table to match a given student, and present that list without a student column, in the UI. It looks like a separate table to the user, but that's not actually how it's stored.
(If you did create a separate guardians table for each student, you'd make it impossible to do queries like ‘find students for a particular guardian’.)