I have two tables:
A table takes with attributes ID and Course_ID
A table course with attributes Course_ID, title, and dept_name
I want to retrieve title and dept_name using a natural join on the two tables, but it returns an error:
incorrect syntax near ';'
My code:
select title, dept_name
from takes
natural join course;
What is wrong with my code?
PS. My textbook does not mention using the on keyword. Besides that, it also mentions to use the operator using to specify the common attributes, but this doesn't help either.
Well standard SQL supports a concept called natural join, which represents an inner
join based on a match between columns with the same name in both sides. For example,
T1 NATURAL JOIN T2 joins the rows between T1 and T2 based on a match between the
columns with the same names in both sides.
T-SQL being a dialect of SQL, doesn’t have an implementation of a natural join, as of SQL Server 2012.
So in your case as takes and course has a common column Course_ID , equivalent representation in T-SQL will be:
select C.title, C.dept_name
from takes T
INNER JOIN course C on C.Course_ID = T.Course_ID;
Sqlserver does not support natural join. Instead of that you can use
INNER JOIN
select c.title, c.dept_name
from takes t
inner join course c on t.Course_ID = c.Course_ID;
Besides that, it also mentions to use the operator using to specify the common attributes, but this doesn't help either.
You either mis-read the textbook or that textbook is plain wrong. natural join does not require any specification on which column to use.
The USING attribute is used for a "regular" join:
select c.title, c.dept_name
from takes t
join course c using (Course_ID);
join is equivalent to inner join. The keyword inner is optional.
join course using (Course_ID) is equivalent to join course c on t.course_id = c.course_id
Related
Exam Question (AQA A-level Computer Science):
[Primary keys shown by asterisks]
Athlete(*AthleteID*, Surname, Forename, DateOfBirth, Gender, TeamName)
EventType(*EventTypeID*, Gender, Distance, AgeGroup)
Fixture(*FixtureID*, FixtureDate, LocationName)
EventAtFixture(*FixtureID*, *EventTypeID*)
EventEntry(*FixtureID*, *EventTypeID*, *AthleteID*)
A list is to be produced of the names of all athletes who are competing in the fixture
that is taking place on 17/09/18. The list must include the Surname, Forename and
DateOfBirth of these athletes and no other details. The list should be presented in
alphabetical order by Surname.
Write an SQL query to produce the list.
I understand that you could do this two ways, one using a WHERE clause and the other using the INNER JOIN clause. However, I am wondering if the order matters when linking the tables.
First exemplar solution:
SELECT Surname, Forename, DateOfBirth
FROM Athlete, EventEntry, Fixture
WHERE FixtureDate = "17/09/2018"
AND Athlete.AthleteID = EventEntry.AthleteID
AND EventEntry.FixtureID = Fixture.FixtureID
ORDER BY Surname
Here is the first exemplar solution, would it still be correct if I was to switch the order of the tables in the WHERE clause, for example:
WHERE FixtureDate = "17/09/2018"
AND EventEntry.AthleteID = Athlete.AthleteID
AND Fixture.FixtureID = EventEntry.FixtureID
I have the same question for the INNER JOIN clause to, here is the second exemplar solution:
SELECT Surname, Forename, DateOfBirth
FROM Athlete
INNER JOIN EventEntry ON Athlete.AthleteID = EventEntry.AthleteID
INNER JOIN Fixture ON EventEntry.FixtureID = Fixture.FixtureID
WHERE FixtureDate = "17/09/2018"
ORDER BY Surname
Again, would it be correct if I used this order instead:
INNER JOIN EventEntry ON Fixture.FixtureID = EventEntry.FixtureID
If the order does matter, could somebody explain to me why it is in the order shown in the examples?
Some advice:
Never use commas in the FROM clause. Always use proper, explicit, standard JOIN syntax.
Use table aliases that are abbreviations for the table names.
Use standard date formats!
Qualify all column names.
Then, the order of the comparisons doesn't matter for equality. I would recommend using a canonical ordering.
So, the query should look more like:
SELECT a.Surname, a.Forename, a.DateOfBirth
FROM Athlete a INNER JOIN
EventEntry ee
ON a.AthleteID = ee.AthleteID INNER JOIN
Fixture f
ON ee.FixtureID = f.FixtureID
WHERE a.FixtureDate = '2018-09-17'
ORDER BY a.Surname;
I am guessing that all the columns in the SELECT come from Athlete. If that is not true, then adjust the table aliases.
There are lots of stylistic conventions for SQL and #gordonlinoff's answer mentions some of the perennial ones.
There are a few answers to your question.
The most important is that (notionally) SQL is a declarative language - you tell it what you want it to do, not how to do it. In a procedural language (like C, or Java, or PHP), the order of execution really matters - the sequence of instructions is part of the procedure. In a declarative language, the order doesn't really matter.
This wasn't always totally true - older query optimizers seemed to like the more selective where clauses earlier on in the statement for performance reasons. I haven't seen that for a couple of decades now, so assume that's not really a thing.
Because order doesn't matter, but correctly understanding the intent of a query does, many SQL developers emphasize readability. That's why we like explicit join syntax, and meaningful aliases. And for readability, the sequence of instructions can help. I favour starting with the "most important" table, usually the one from which you're selecting most columns, and then follow a logical chain of joins from one table to the next. This makes it easier to follow the logic.
When you use inner joins order does not matter as long as the prerequisite table is above/before. At your example both joins start from table Athlete so order doesn't matter. If however this very query is found starting from EventEntry (for any reason), then you must join at Athlete at the first inner else you cannot join to Fixture. As recommended, it is best to use standard join syntax and preferable place all inner joins before all lefts. If you cant then you need to review because the left you need to put inside the group of inner joins will probably behave like an inner join. That is because an inner below uses the left table else you could place it below the inner block. So when it comes to null the left will be ok but the inner below will cut the record.
When however the above cases do not exist/affect order and all inner joins can be placed at any order, only performance matters. Usually table with high cardinality on top perform better while there are cases where the opposite works better. So if the order is free you may try higher to lower cardinality tables ordering or the opposite - whatever works faster.
Clarifying: As prerequisite table i call the table needed by the joined table by condition: ... join B on [whatever] join C on c.id=b.cid - here table B is prerequisite for table C.
I mention left joins because while the question is about inner order, when joins are mixed (inners and lefts)then order of joins alone is important (to be all above) as may affect query logic:
... join B on [whatever] left join C on c.id=b.cid join D on D.id = C.did
At the above example the left join sneaks into the inner joins order. We cannot order it after D because it is prerequisite for D. For records however where condition c.id=b.cid is not true the entire B table row turns null and then the entire result row (B+C+D) turns off the results because of D.id = C.did condition of the following inner join. This example needs review as the purpose of left join evaporates by the following (next on order) inner join. Concluding, the order of inner joins when mixed with lefts is better to be on top without any left joins interfering.
What's the differences between these SQLs:
SELECT *
FROM COURS NATURAL JOIN COMPOSITIONCOURS NATURAL JOIN PARCOURS;
SELECT *
FROM COURS NATURAL JOIN (COMPOSITIONCOURS C JOIN PARCOURS P ON C.IDPARCOURS = P.IDPARCOURS) ;
SELECT *
FROM (COURS NATURAL JOIN COMPOSITIONCOURS C) JOIN PARCOURS P ON C.IDPARCOURS = P.IDPARCOURS ;
They have different results.
It's difficult to tell precisely without a sample result set, but I would imagine its your utilization of natural JOINs which is typically bad practice. Reasons being:
I would avoid using natural joins, because natural joins are:
Not standard SQL.
Not informative. You aren't specifying what columns are being joined without referring to the schema.
Your conditions are vulnerable to schema changes. If there are multiple natural join columns and one such column is removed from a table, the query will still execute, but probably not correctly and this change in behavior will be silent.
Thanks for all of yours answers! I finally find out that the problem is because of the use of natural JoIn.
When using natural join, it will join every column with the same name! When I am using NATURAL JOIN for several times , it's possible those columns, which have same names but you don't wants to combine , could be joined automatically!
Here is an example,
Table a (IDa, name,year,IDb)
Table b (IDb, bonjour,salute,IDc)
Table c (IDc, year, merci)
If I wrote like From a Natural Join b Natural c,because IDb and IDc are common names for Table a,b,c. It seems Ok! But attention please!
a.year and b.year can also be joined ! That's what we don't want!
I have been googled some for an explanation of the SQL function ON though
I couldn't find a good explanation how it work.
is it associated/connected to INNER JOIN?
Could someone please explain my Code-snippet what really happens?
(see my code below)
SELECT
TS_TEST_ID as Test_ID,
TS_NAME as Name
FROM TEST
INNER JOIN DESSTEPS
ON TEST.TS_TEST_ID = DESSTEPS.DS_TEST_ID
INNER JOIN ALL_LISTS
ON ALL_LISTS.AL_ITEM_ID = TEST.TS_SUBJECT
It is not a function, it is part of language. Like with natural language you have various types of words: like nouns, verbs etc. This is like proposition.
ON is a part of syntax for INNER JOIN, it goes like this:
one table INNER JOIN some other table ON how do I want to join both tables (key columns)
You might find some more details here
on tells the join with which condition the tables should be connected.
In this case:
FROM TEST
INNER JOIN DESSTEPS
ON TEST.TS_TEST_ID = DESSTEPS.DS_TEST_ID
You tables test will be joined on column TS_TEST_ID and DS_TEST_ID. So records belong together, where These id's are equals
I'm studying SQL for a database exam and the way I've seen SQL is they way it looks on this page:
http://en.wikipedia.org/wiki/Star_schema
IE join written the way Join <table name> On <table attribute> and then the join condition for the selection. My course book and my exercises given to me from the academic institution however, use only natural join in their examples. So when is it right to use natural join? Should natural join be used if the query can also be written using JOIN .. ON ?
Thanks for any answer or comment
A natural join will find columns with the same name in both tables and add one column in the result for each pair found. The inner join lets you specify the comparison you want to make using any column.
IMO, the JOIN ON syntax is much more readable and maintainable than the natural join syntax. Natural joins is a leftover of some old standards, and I try to avoid it like the plague.
A natural join will find columns with the same name in both tables and add one column in the result for each pair found. The inner join lets you specify the comparison you want to make using any column.
The JOIN keyword is used in an SQL statement to query data from two or more tables, based on a relationship between certain columns in these tables.
Different Joins
* JOIN: Return rows when there is at least one match in both tables
* LEFT JOIN: Return all rows from the left table, even if there are no matches in the right table
* RIGHT JOIN: Return all rows from the right table, even if there are no matches in the left table
* FULL JOIN: Return rows when there is a match in one of the tables
INNER JOIN
http://www.w3schools.com/sql/sql_join_inner.asp
FULL JOIN
http://www.w3schools.com/sql/sql_join_full.asp
A natural join is said to be an abomination because it does not allow qualifying key columns, which makes it confusing. Because you never know which "common" columns are being used to join two tables simply by looking at the sql statement.
A NATURAL JOIN matches on any shared column names between the tables, whereas an INNER JOIN only matches on the given ON condition.
The joins often interchangeable and usually produce the same results. However, there are some important considerations to make:
If a NATURAL JOIN finds no matching columns, it returns the cross
product. This could produce disastrous results if the schema is
modified. On the other hand, an INNER JOIN will return a 'column does
not exist' error. This is much more fault tolerant.
An INNER JOIN self-documents with its ON clause, resulting in a
clearer query that describes the table schema to the reader.
An INNER JOIN results in a maintainable and reusable query in
which the column names can be swapped in and out with changes in the
use case or table schema.
The programmer can notice column name mis-matches (e.g. item_ID vs itemID) sooner if they are forced to define the ON predicate.
Otherwise, a NATURAL JOIN is still a good choice for a quick, ad-hoc query.
I've never learned how joins work but just using select and the where clause has been sufficient for all the queries I've done. Are there cases where I can't get the right results using the WHERE clause and I have to use a JOIN? If so, could someone please provide examples? Thanks.
Implicit joins are more than 20 years out-of-date. Why would you even consider writing code with them?
Yes, they can create problems that explicit joins don't have. Speaking about SQL Server, the left and right join implicit syntaxes are not guaranteed to return the correct results. Sometimes, they return a cross join instead of an outer join. This is a bad thing. This was true even back to SQL Server 2000 at least, and they are being phased out, so using them is an all around poor practice.
The other problem with implicit joins is that it is easy to accidentally do a cross join by forgetting one of the where conditions, especially when you are joining too many tables. By using explicit joins, you will get a syntax error if you forget to put in a join condition and a cross join must be explicitly specified as such. Again, this results in queries that return incorrect values or are fixed by using distinct to get rid of the cross join which is inefficient at best.
Moreover, if you have a cross join, the maintenance developer who comes along in a year to make a change doesn't know if it was intended or not when you use implicit joins.
I believe some ORMs also now require explicit joins.
Further, if you are using implied joins because you don't understand how joins operate, chances are high that you are writing code that, in fact, does not return the correct result because you don't know how to evaluate what the correct result would be since you don't understand what a join is meant to do.
If you write SQL code of any flavor, there is no excuse for not thoroughly understanding joins.
Yes. When doing outer joins. You can read this simple article on joins. Joins are not hard to understand at all so you should start learning (and using them where appropriate) right away.
Are there cases where I can't get the right results using the WHERE clause and I have to use a JOIN?
Any time your query involves two or more tables, a join is being used. This link is great for showing the differences in joins with pictures as well as sample result sets.
If the join criteria is in the WHERE clause, then the ANSI-89 JOIN syntax is being used. The reason for the newer JOIN syntax in the ANSI-92 format, is that it made LEFT JOIN more consistent across various databases. For example, Oracle used (+) on the side that was optional while in SQL Server you had to use =*.
Implicit join syntax by default uses Inner joins. It is sometimes possible to modify the implicit join syntax to specify outer joins, but it is vendor dependent in my experience (i know oracle has the (-) and (+) notation, and I believe sqlserver uses *= ). So, I believe your question can be boiled down to understanding the differences between inner and outer joins.
We can look at a simple example for an inner vs outer join using a simple query..........
The implicit INNER join:
select a.*, b.*
from table a, table b
where a.id = b.id;
The above query will bring back ONLY rows where the 'a' row has a matching row in 'b' for it's 'id' field.
The explicit OUTER JOIN:
select * from
table a LEFT OUTER JOIN table b
on a.id = b.id;
The above query will bring back EVERY row in a, whether or not it has a matching row in 'b'. If no match exists for 'b', the 'b' fields will be null.
In this case, if you wanted to bring back EVERY row in 'a' regardless of whether it had a corresponding 'b' row, you would need to use the outer join.
Like I said, depending on your database vendor, you may still be able to use the implicit join syntax and specify an outer join type. However, this ties you to that vendor. Also, any developers not familiar wit that specialized syntax may have difficulty understanding your query.
Any time you want to combine the results of two tables you'll need to join them. Take for example:
Users table:
ID
FirstName
LastName
UserName
Password
and Addresses table:
ID
UserID
AddressType (residential, business, shipping, billing, etc)
Line1
Line2
City
State
Zip
where a single user could have his home AND his business address listed (or a shipping AND a billing address), or no address at all. Using a simple WHERE clause won't fetch a user with no addresses because the addresses are in a different table. In order to fetch a user's addresses now, you'll need to do a join as:
SELECT *
FROM Users
LEFT OUTER JOIN Addresses
ON Users.ID = Addresses.UserID
WHERE Users.UserName = "foo"
See http://www.w3schools.com/Sql/sql_join.asp for a little more in depth definition of the different joins and how they work.
Using Joins :
SELECT a.MainID, b.SubValue AS SubValue1, b.SubDesc AS SubDesc1, c.SubValue AS SubValue2, c.SubDesc AS SubDesc2
FROM MainTable AS a
LEFT JOIN SubValues AS b ON a.MainID = b.MainID AND b.SubTypeID = 1
LEFT JOIN SubValues AS c ON a.MainID = c.MainID AND b.SubTypeID = 2
Off-hand, I can't see a way of getting the same results as that by using a simple WHERE clause to join the tables.
Also, the syntax commonly used in WHERE clauses to do left and right joins (*= and =*) is being phased out,
Oracle supports LEFT JOIN and RIGHT JOIN using their special join operator (+) (and SQL Server used to support *= and =* on join predicates, but no longer does). But a simple FULL JOIN can't be done with implicit joins alone:
SELECT f.title, a.first_name, a.last_name
FROM film f
FULL JOIN film_actor fa ON f.film_id = fa.film_id
FULL JOIN actor a ON fa.actor_id = a.actor_id
This produces all films and their actors including all the films without actor, as well as the actors without films. To emulate this with implicit joins only, you'd need unions.
-- Inner join part
SELECT f.title, a.first_name, a.last_name
FROM film f, film_actor fa, actor a
WHERE f.film_id = fa.film_id
AND fa.actor_id = a.actor_id
-- Left join part
UNION ALL
SELECT f.title, null, null
FROM film f
WHERE NOT EXISTS (
SELECT 1
FROM film_actor fa
WHERE fa.film_id = f.film_id
)
-- Right join part
UNION ALL
SELECT null, a.first_name, a.last_name
FROM actor a
WHERE NOT EXISTS (
SELECT 1
FROM film_actor fa
WHERE fa.actor_id = a.actor_id
)
This will quickly become very inefficient both syntactically as well as from a performance perspective.