This question already has answers here:
What is the difference between "INNER JOIN" and "OUTER JOIN"?
(28 answers)
Closed 5 years ago.
Order Table
Product Table
I need to select all from these two tables. Is it possible?
With the following query you can select all data from one table:
SELECT * FROM Order;
If you want to select all data from two tables you can use a join in the select query to connect the data of the two tables:
SELECT * FROM Order
JOIN Product ON Order.p_id = Product.p_id;
There are different kinds of joins available. Based on the one you use can change the amount of data you will receive. The different kinds of joins are:
(Inner) join: Returns records that have matching values in both tables
Left (Outer) join: Return all records from the left table, and the matched records from the right table
Right (outer) join: Return all records from the right table, and the matched records from the left table
Full (outer) join: Return all records when there is a match in either left or right table
The information about the different kinds of joins comes from the following website:https://www.w3schools.com/sql/sql_join.asp
Related
I have 3 tables of data I'm joining together. I'm using an ID column (day) in each table to join the data together. The following SQL query works, and provides the following column set:
SELECT *
FROM records
LEFT JOIN macro_nutrients ON macro_nutrients.day = records.day
LEFT JOIN micro_nutrients ON micro_nutrients.day = records.day
day|date|calories_in|calories_out|day|fat|sugar|protein|day|iron|cholesterol|potassium|vitamin_a|vitamin_c|
As I join tables, the "day" field (which works as my primary key for joining tables) gets outputted each time (3 tables would have three day fields listed). I would like to exclude the day fields from my column set, as they aren't needed.
Would this only be achievable by explicitly mentioning the fields I want in my select statement (eg. macro_nutrients.fat)? this would unfortunately get quite verbose if so. Curious if there a more compact way.
You can use USING:
SELECT *
FROM records r LEFT JOIN
macro_nutrients man
USING (day) LEFT JOIN
micro_nutrients mn
USING (day);
USING is SQL standard syntax that both SQLite and MySQL support. When * is used with USING, the columns used as JOIN keys are not duplicated in the result set. In an outer join, the value is the actual value in the data -- rather than the unmatched NULL value.
This question already has answers here:
Filter Table Before Applying Left Join
(4 answers)
Closed 2 years ago.
I want to join two tables based on the below scenarios:
Need to display all the requirements table values
Join the second table(prod_spec) based on the requirement_id and inquiry_id.
Table 1(requirements):
Table 2(prod_spec):
Using below query I'm getting the result below:
SELECT requirements.id,requirements.requirement, prod_spec.evidence,prod_spec.status,prod_spec.no_specify,prod_spec.inquiry_id FROM requirements LEFT JOIN product_spec ON prod_spec.requirement_id=requirement.id
The problem is not getting any result when I put the where condition eg: where inquiry_id='67' to the sql query. Please let me know how to display the rows based on inquiry_id.
FYI, requirement_id will only there once in prod_spec table based on inquiry_id.
The problem is not getting any result when I put the where condition eg: where inquiry_id = '67' to the sql query
The condition on the LEFT JOINed table needs to go to the ON clause of the JOIN:
SELECT r.id,r.requirement, ps.evidence,ps.status, ps.no_specify, ps.inquiry_id
FROM requirements r
LEFT JOIN prod_spec
ON ps.requirement_id = requirement.id
AND ps.inquiry_id = 67 --> here
Rationale: if you put that condition in the WHERE clause, then it becomes mandatory; as a consequence, rows from requirements for which there is no match in prod_spec are evicted from the resultset.
Side notes:
table aliases make the query easier to write and read
it seems like inquiry_id is a number, so it should be compared as such; don't put single quotes around literal 67
This question already has answers here:
What's the difference between comma separated joins and join on syntax in MySQL? [duplicate]
(5 answers)
Closed 6 years ago.
Is there any performance difference between these two query. Both query returns the same result.
select * from tbl_a, tbl_b
and
select * from tbl_a INNER JOIN tbl_b
There is a thread already touching on this topic. Check it out:
What is the difference between inner join and outer join
Performance wise, it will all come down to the amount of data you're working with. A couple tables with a few hundred rows won't make a dent, but you will notice a difference as the amount of data sorted grows.
Good luck!
i have two tables as below:
Table 1 "customer" with fields "Cust_id", "first_name", "last_name" (10 customers)
Table 2 "cust_order" with fields "order_id", "cust_id", (26 orders)
I need to display "Cust_id" "first_name" "last_name" "order_id"
to where i need count of order_id group by cust_id like list total number of orders placed by each customer.
I am running below query, however, it is counting all the 26 orders and applying that 26 orders to each of the customer.
SELECT COUNT(order_id), cus.cust_id, cus.first_name, cus.last_name
FROM cust_order, customer cus
GROUP BY cust_id;
Could you please suggest/advice what is wrong in the query?
You issue here is that you have told the database how these two tables are 'connected', or what they should be connected by:
Have a look at this image:
~IMAGE SOURCE
This effectively allows you to 'join' two tables together, and use a query between them.
so you might want to use something like:
SELECT COUNT(B.order_id), A.cust_id, A.first_name, A.last_name
FROM customer A
LEFT JOIN cust_order B //this is using a left join, but an inner may be appropriate also
ON (A.cust_id= B.Cust_id) //what links them together
GROUP BY A.cust_id; // the group by clause
As per your comment requesting some further info:
Left Join (right joins are almost identical, only the other way around):
The SQL LEFT JOIN returns all rows from the left table, even if there are no matches in the right table. This means that if the ON clause matches 0 (zero) records in right table, the join will still return a row in the result, but with NULL in each column from right table. ~Tutorials Point.
This means that a left join returns all the values from the left table, plus matched values from the right table or NULL in case of no matching join predicate.
LEFT joins will be used in the cases where you wish to retrieve all the data from the table in the left hand side, and only data from the right that match.
Execution Time
While the accepted answer in this case may work well in small datasets, it may however become 'heavy' in larger databases. This is because it was not actually designed for this type of operation.
This was the purpose of Joins to be introduced.
Much work in database-systems has aimed at efficient implementation of joins, because relational systems commonly call for joins, yet face difficulties in optimising their efficient execution. The problem arises because inner joins operate both commutatively and associatively. ~Wikipedia
In practice, this means that the user merely supplies the list of tables for joining and the join conditions to use, and the database system has the task of determining the most efficient way to perform the operation. A query optimizer determines how to execute a query containing joins. So, by allowing the dbms to choose the way your data is queried, you can save a lot of time.
Other Joins/Summary
AN INNER JOIN will return data from both tables where the keys in each table match
A LEFT JOIN or RIGHT JOIN will return all the rows from one table and matching data from the other table.
Use a join when you want to query multiple tables.
Joins are much faster than other ways of querying >=2 tables (speed can be seen much better on larger datasets).
You could try this one:
SELECT COUNT(cus_order.order_id), cus.cust_id, cus.first_name, cus.last_name
FROM cust_order cus_order, customer cus
WHERE cus_order.cust_id = cus.cust_id
GROUP BY cust_id;
Maybe an left join will help you
SELECT COUNT(order_id), cus.cust_id, cus.first_name, cus.last_name ]
FROM customer cus
LEFT JOIN cust_order co
ON (co.cust_id= cus.Cust_id )
GROUP BY cus.cust_id;
This question already has answers here:
Joining two tables in a MySQL
(2 answers)
Closed 8 years ago.
I want to select all from a table called Customer where CustomerID = 'AC001' and I also want to join in another table Pets where the CustomerID(fk) is the same as the CustomerID in Customer table is that possible?
Customer table
CustomerID,TypeOfCustomer, FName,Adress,City,State,Zipcode,PhoneNr,FaxNr
Pets table
PetID,FName, Animal,Breed,Gender,DoB,CustomerID(fk)
This is a very basic SQL question, here you go: to join the two tables together, do this:
SELECT *
FROM Customer
JOIN Pets ON Pets.CustomerId=Customer.CustomerID
To filter for CustomerID = 'AC001', add a WHERE clause.
Note also that the result won't contain customers without any pets. If you want to include those as well, do a LEFT JOIN instead.
Generally, I'd recommend reading a good beginner-level book on SQL and relational databases to make sure you grasp the underlying concepts here.
Select //fields you want// from Customer C, Pets P
WHERE C.CustomerID=P.CustomerID