What I meant was, there's a Table A having 5 columns. I have a SP that I use to get 3 columns from Table A and one column from Table B. Now, would it be better to add the column from Table B to Table A or use a sub-query in that SP to get that column from Table B?
Your question is still confusing and it very much looks like you haven't understood how to use a relational database. So let me try to explain:
Let's say you have two tables:
client
client_number
first_name
last_name
date_of_birth
...
order
order_number
client_number
order_date
...
These are two separate tables, so as to have a normalized relational database. The order table contains the client number, so you can look up the clients name and date of birth in the client table. The date of birth may be important in order know whether the client is allowed to order certain articles. But you don't want to store the date of birth with every order - it doesn't change.
If you want to look up the age you can use a sub query:
select
order_number,
order_date,
quantity,
(
select date_of_birth
from client c
where c.client_number = o.client_number
)
from order o
where item = 'whisky';
but most often you would simply join the tables:
select
o.order_number,
o.order_date,
o.quantity,
c.date_of_birth,
c.first_name,
c.last_name
from order o
join client c on c.client_number = o.client_number;
You would however not change your tables and invite redundancy with all its problems only not to have to join. You design your database such that it is a well-formed relational database, not such that it makes your latest query easy to write. It is very, very common to use joins and subqueries, and having to use them usually shows that you built your database well.
I think this a good time you look up database normalization, e.g. in Wikipedia https://en.wikipedia.org/wiki/Database_normalization.
Related
Made this account just to ask about this question after being unable to find/expending the local resources I have, so I come to you all.
I'm trying to join two tables - ORDERS and CUSTOMER - as per a question on my assignment
For every order, list the order number and order date along with the customer number, last name, and first name of the customer who placed the order.
So I'm looking for the order number, date, customer number, and the full name of customers.
The code goes as such
SELECT ORDERS.ORDR_ORDER_NUMBER, ORDERS.ORDR_ORDER_DATE, ORDERS.ORDR_CUSTOMER_NUMBER, CUSTOMER.CUST_LAST, CUSTOMER.CUST_FIRST
FROM ORDERS, CUSTOMER
WHERE ORDERS.ORDR_CUSTOMER_NUMBER = CUSTOMER.CUST_CUSTOMER_NUMBER;
I've done this code without the table identifiers, putting quotation marks around ORDERS.ORDR_CUSTOMER_NUMBER, aliases for the two tables, and even putting a space after ORDR_ in both SELECT & WHERE for laughs and nothing's working. All of them keep coming up with the error in the title (ORA-00904), saying [ORDERS.]ORDR_CUSTOMER_NUMBER is the invalid identifier even though it shouldn't be.
Here also are the tables I'm working with, in case that context is needed for help.
Anyway, the query that produces the result you want should take the form:
select
o.ordr_order_number,
o.ordr_order_date,
c.cust_customer_number,
c.cust_last,
c.cust_first
from orders o
join customer c on c.cust_customer_number = o.ordr_customer_number
As you see the query becomes a lot easier to read and write if you use modern join syntax, and if you use table aliases (o and c).
You have to add JOIN or INNER JOIN to your query. Because the data comes from two different tables the WHERE clause will not select both.
FROM Orders INNER JOIN Customers ON Orders.order_Customer_Number = Customer.Cust_Customer_Number
I currently have three tables.
master_tradesmen
trades
master_tradesmen_trades (joins the previous two together in a many-to-many relationship). The 'trade_id' and 'master_tradesman_id' are the foreign keys.
Here is what I need to happen. A user performs a search and types in a trade. I need a query that displays all of the information from the master_tradesmen table whose trade in the master_tradesmen_trade table matches the search. For example, if 'plumbing' is typed in the search bar (trade_id 1), all of the columns for Steve Albertsen (master_tradesman_id 6) and Brian Terry (master_tradesman_id 8) would be displayed from the master_tradesmen table. As a beginner to SQL, trying to grasp the logic of this is about to make my head explode. I'm hoping that someone with more advanced SQL knowledge can wrap their head around this much easier than I can. Note: the 'trades' column in master_tradesmen is for display purposes only, not for querying. Thank you so much in advance!
You have a catalog for the tradesmen, & another catalog for the trades.
The trades should only appear once in the trades catalog in order to make your DB more consistent
Then you have your many-to-many table which connects the trades & master tradesmen tables.
If we want to get the tradesmen according to the given trade in the input, we should first
know the id of that trade which has to be unique, so in your DB you would have something
like the img. below :
Now we can make a query to select the id of trade :
DECLARE #id_trade int = SELECT trade_id FROM trades WHERE trade_name LIKE '%plumbing%'
Once we know the trading id, we can redirect to the 'master_tradesmen_trades' table to know the name of the people how work that trade :
SELECT * FROM master_tradesmen_trades WHERE trade_id = #id_trade
You will get the following result :
You may say, 'But there is still something wrong with it, as i am still not able to see the tradesmen', this is the moment when we make an inner join:
SELECT * FROM master_tradesmen_trades trades_and_tradesmen
INNER JOIN master_tradesman tradesmen
ON tradesmen.id = trades_and_tradesmen.master_tradesmen_id
WHERE trade_id = #id_trade
IF you need to see specific columns, you can do :
SELECT first_name, last_name, city, state FROM master_tradesmen_trades trades_and_tradesmen
INNER JOIN master_tradesman tradesmen
ON tradesmen.id = trades_and_tradesmen.master_tradesmen_id
WHERE trade_id = #id_trade
I am new to sql and have a question about joining 2 tables. Why is there a . in between customers.custnum in this example. What is its significance and what does it do?
Ex.
Select
customers.custnum, state, qty
From
customers
Inner join
sales On customers.custnum = sales.custnum
The . is to specify a column of a table.
Let's use your customer table; we could do:
SELECT c.custnum, c.state, c.qty FROM customers as c INNER JOIN
sales as s ON c.custnum = s.custnum
You don't really need the . unless two tables have columns with the same name.
In the below query, there are two tables being referred. One is CUSTOMERS another is STATE. Since both has same column CUSTNUM, we need a way to tell the database which CUSTNUM are we referring to. Same as there may be many Bob's, if so their last name is used for disambiguation.
I would consider the below style as more clearer. That's opinionated.
Select
cust.custnum, cust.state, s.qty
From
customers cust -- use alias for meaningful referencing, you may be self-joining, during that time you can use cust1, cust2 as aliases.
Inner join
sales as s On cust.custnum = s.custnum
Think of it as a way to categorize the hierarchical nature of the database. Within a DB, there are tables, and within tables there are columns. It's just a way of keeping track, especially if you are working with multiple tables that may have the same column name.
For example, a table called Sales and a table called Customers might both have a column called Date. You may be writing a query where you only want the date from the Sales table, so you would specify that by writing:
Select *
From Sales
inner join Customers on Sales.ID = Customers.ID
where Sales.Date = '1/1/2019'
I'm pretty sure this works universally across various SQL implementations. Suppose I have many-to-many relationship between 2 tables:
Customer: id, name
has many:
Order: id, description, total_price
and this relationship is in a junction table:
Customer_Order: order_date, customer_id, order_id
Now I want to write SQL query to join all of these together, mentioning the customer's name, the order's description and total price and the order date:
SELECT name, description, total_price FROM Customer
JOIN Customer_Order ON Customer_Order.customer_id = Customer.id
JOIN Order = Order.id = Customer_Order.order_id
This is all well and good. This query will also work if we change the order so it's FROM Customer_Order JOIN Customer or put the Order table first. Why is this the case? Somewhere I've read that JOIN works like an arithmetic operator (+, * etc.) taking 2 operands and you can chain operator together so you can have: 2+3+5, for example. Following this logic, first we have to calculate 2+3 and then take that result and add 5 to it. Is it the same with JOINs?
Is it that behind the hood, the first JOIN must first be completed in order for the second JOIN to take place? So basically, the first JOIN will create a table out of the 2 operands left and right of it. Then, the second JOIN will take that resulting table as its left operand and perform the usual joining. Basically, I want to understand how multiple JOINs work behind the hood.
In many ways I think ORMs are the bane of modern programming. Unleashing a barrage of underprepared coders. Oh well diatribe out of the way, You're asking a question about set theory. THere are potentially other options that center on relational algebra but SQL is fundamentally set theory based. here are a couple of links to get you started
Using set theory to understand SQL
A visual explanation of SQL
I have two tables PRODUCT and ACCOUNT.
PRODUCT table columns
product_id (PK)
subscription_id
ACCOUNT table columns
account_nbr
subscription_id
(account_nbr and subscription_id are primary key columns in account table)
... Other columns
I have to find account_nbr and subscription_id for a product_id.
I get product_id as input. Using it I can get the subscription_id from PRODUCT table and
using subscription_id I can get the account_nbr value from ACCOUNT table.
Instead of getting the info in two queries, can it be done in one query?
Something like the below:
select distinct a.acct_nbr,p.subscription_id
from ACCOUNT a,PRODUCT p
where v.product_id = ' val 1' and
v.subscription_id = p.subscription_id
Will the performance of the above query be low compared to two separate queries?
I have to find account_nbr and subscription_id for a product_id.
So, you're correct in your approach you need to JOIN the two result-sets together:
select p.account_nbr, p.subscription_id
from account a
join product p
on a.subscription_id = p.subscription_id
where p.product_id = :something
Some points:
You have an alias v in your query; I don't know where this came from.
Learn to use the ANSI join syntax; if you make a mistake it's a lot more obvious.
You're selecting a.acct_nbr, which doesn't exist as-is.
There's no need for a DISTINCT. ACCOUNT_NBR and PRODUCT_ID are the primary key of ACCOUNT.
Will the performance of the above query be low compared to two separate queries?
Probably not. If your query is correct and your tables are correctly indexed it's highly unlikely that whatever you've coded could beat the SQL engine. The database is designed to select data quickly.