I prefer to indicate join types when I use it in database systems but when I switch to a new project, there is a single join is used. Generally I prefer to use LEFT JOIN or INNER JOIN according to my needs, but I have not found which JOIN type is considered when a single JOIN is used in PostgreSQL.
select p.uuid from Product s " +
join Category c on p.uuid = c.siteUuid
join Brand b on b.uuid = c.brandUuid
Inner Join is the default join when we use plain JOIN.
For better readablity of the queries, It is always preferred to write INNER JOIN
Reference:
https://www.postgresql.org/docs/current/queries-table-expressions.html#id-1.5.6.6.5.6.4.3.1.2
Related
I have some queries I need migrate to Ecto and for maintainability reasons, I'd rather not just wrap them in a fragment and call it a day.
They have a lot of LEFT JOINs in them and as I understand from this answer, a left_join in Ecto does a LEFT OUTER JOIN by default. I can't seem to figure out how to specify to Ecto that I want a LEFT INNER JOIN, which is the default behavior for a LEFT JOIN in Postgresql.
To look at a toy example, let's say posts in our database can be either anonymous or they can have a creator. I have a query to get just enough info to make a post preview, but I only want non-anonymous posts to be included:
SELECT
p.id,
p.title,
p.body,
u.name AS creator_name,
u.avatar AS creator_avatar,
FROM posts p
LEFT JOIN users u ON p.creator_id = u.id;
I would translate that into Ecto as:
nonanonymous_posts =
from p in Post,
left_join: u in User, on: p.creator_id == u.id,
select: [p.id, p.title, p.body, u.name, u.avatar]
and Ecto spits out
SELECT
t0."id",
t0."title",
t0."body",
t1."name" AS creator_name,
t1."avatar" AS creator_avatar,
FROM "posts" AS t0
LEFT OUTER JOIN "users" as t1 ON t0."creator_id" = t1."id";
which will give back anonymous posts as well.
There is no such thing as LEFT INNER JOIN. There is only INNER JOIN and LEFT [OUTER] JOIN (OUTER part is optional, as LEFT JOIN must be outer join). So what you want is just :join or :inner_join in your Ecto query.
We are trying to replace source qualifier override with user defined joins and source filter.
for below user defined join in Informatica source qualifier:
{A INNER JOIN B ON a.dept_id= b.dept_id
b.load_date between 20170712174712000000 and 20170904152656000000
LEFT OUTER JOIN C ON a.emp_id = c.emp_id}
for this I'm getting SQL query as
FROM A,B,C WHERE {A INNER JOIN B ON A.dept_id = B.dept_id
AND b.load_date between 20170712174712000000 and 20170904152656000000
LEFT OUTER JOIN C
ON C ON a.emp_id = c.emp_id}
I have tried replacing INNER JOIN in override query with NORMAL JOIN, as I saw it somewhere that informatica translates normal to inner join.
The source database is DB2.
I don't know anything about informatica, but the resulting SQL syntax that you listed in the question is not valid for DB2. The biggest problem is that you have the JOIN in the WHERE clause rather than the FROM clause. Not real sure how to fix that in informatica though. Appropriate DB2 syntax though would be something like this:
FROM a
INNER JOIN b ON a.dept_id = b.dept_id
LEFT OUTER JOIN c ON a.emp_id = c.emp_id
WHERE
b.load_date BETWEEN 20170712174712000000 and 20170904152656000000
This assumes that b.load_date is not a timestamp field. If you are using a timestamp field, the format for the timestamps should be '2017-07-12 17:47:12.000000'
The T-SQL should be used in FROM clause not in the WHERE clause. Hence use:
FROM A INNER JOIN B ON a.dept_id = b.dept_id
LEFT OUTER JOIN C ON a.emp_id = c.emp_id
I want to join two tables together and add additional information from two other tables to the same columns in both queried tables. I've come up with the below code, which works, but I don't feel comfortable about having to add another JOIN clause for each table, as it would make the query substantially long if I wanted to join/add more things.
Is there a way to combine it, so that I can join additional tables only once (just use S and E aliases every time)?
SELECT
J.JobId,
J.StandardJobId,
S.JobName,
J.EngineerId,
E.EngineerName,
JF.JobId AS FollowUpJobId,
JF.StandardJobId AS FollowUpStandardJobId,
SF.JobName AS FollowUpJobName,
JF.EngineerId AS FollowUpEngineerId,
EF.EngineerName AS FollowUpEngineerName
FROM
Jobs J
INNER JOIN
Jobs JF
ON
J.FollowUpJobId = JF.JobId
INNER JOIN
StandardJobs S
ON
J.StandardJobId = S.StandardJobId
INNER JOIN
Engineers E
ON
E.EngineerId = J.EngineerId
INNER JOIN
StandardJobs SF
ON
SF.StandardJobId = JF.StandardJobId
INNER JOIN
Engineers EF
ON
EF.EngineerId = JF.EngineerId
One approach would be to use a Common Table Expression (CTE) - something like:
with cte as
(SELECT J.JobId,
J.StandardJobId,
S.JobName,
J.EngineerId,
E.EngineerName,
J.FollowUpJobId
FROM Jobs J
INNER JOIN StandardJobs S ON J.StandardJobId = S.StandardJobId
INNER JOIN Engineers E ON E.EngineerId = J.EngineerId)
SELECT O.*,
F.StandardJobId AS FollowUpStandardJobId,
F.JobName AS FollowUpJobName,
F.EngineerId AS FollowUpEngineerId,
F.EngineerName AS FollowUpEngineerName
FROM CTE AS O
JOIN CTE AS F ON O.FollowUpJobId = F.JobId
You can sort of do this with either a CTE (Common Table Expressions, the WITH clause) or a View:
;WITH Jobs_Extended As
(
SELECT j.*,
s.JobName,
E.EngineerName
FROM Jobs As j
JOIN StandardJobs As s ON s.StandardJobId = j.StandardJobId
JOIN Engineer As e ON e.EngineerId = j.EngineerId
)
SELECT
J.JobId,
J.StandardJobId,
J.JobName,
J.EngineerId,
J.EngineerName,
JF.JobId AS FollowUpJobId,
JF.StandardJobId AS FollowUpStandardJobId,
JF.JobName AS FollowUpJobName,
JF.EngineerId AS FollowUpEngineerId,
JF.EngineerName AS FollowUpEngineerName
FROM Jobs_Extended J
JOIN Jobs_Extended JF ON J.FollowUpJobId = JF.JobId
In this example the CTE Jobs_Extended becomes a defined alias for the relationship between the Jobs, Engineers and StandardJobs tables. Then once defined, you can use it multiple times in the query without having to redefine those interior relations.
You can do the same thing by change the WITH to a View, which will make the defined alias permannet in your database.
No, you cannot avoid JOINing related tables each time a separate reference is needed. The issue is that you are not working with the tables in a general sense but instead working with the specific rows of each table, even more specifically, just those rows that match the JOIN and WHERE conditions.
There is no way to specify the references to either StandardJobs or Engineers only once because you are needing to work with two rows from each table at the same time, at least in the given example.
However, depending on which direction you are wanting to go with "additional tables" (more references to Jobs or more lookups like StandardJobs and Engineers for the given 2 references of Jobs), the CTE construct shown by Mark is the probably the easiest / best way to abstract it. I posted this answer mainly to explain the issue at hand.
First Query:
Select * from table1 inner join table2 on table1.Id = table2.Id
Second Query:
Select * from table1, table2 where table1.Id = table2.Id
What is difference between these query regarding performance which should one use?
The two statements you posted are logically identical. There isn't really a
practical reason to prefer one over the other, it's largely a matter of
personal style and readability. Some people prefer the INNER JOIN syntax and
some prefer just to use WHERE.
Refering to Using Inner Joins:
In the ISO standard, inner joins can
be specified in either the FROM or
WHERE clause. This is the only type of
join that ISO supports in the WHERE
clause. Inner joins specified in the
WHERE clause are known as old-style
inner joins.
Refering to Join Fundamentals:
Specifying the join conditions in the
FROM clause helps separate them from
any other search conditions that may
be specified in a WHERE clause, and is
the recommended method for specifying
joins.
Personaly, I prefer using INNER JOIN. I find it much clearer, as I can separate the join conditions from the filter conditions and using a seperate join block for each joined table.
To amplify #Akram's answer - many people prefer the inner join syntax, since it then allows you to more easily distinguish between the join conditions (how the various tables in the FROM clause relate to each other) from the filter conditions (those conditions that should be used to reduce the overall result set. There's no difference between them in this circumstance, but on larger queries, with more tables, it may improve readability to use the inner join form.
In addition, once you start considering outer joins, you pretty well need to use the infix join syntax (left outer join,right outer join), so many find a form of symmetry in using the same style for inner join. There is an older deprecated syntax for performing outer joins in the WHERE clause (using *=), but support for such joins is dying out.
I have a fairly conventional set of order entry tables divided by:
Orders
OrdersRows
OrdersRowsOptions
The record in OrderRowOptions is not created unless needed. When I create a set of joins like
select * from orders o
inner join OrdersRows r on r.idOrder = o.idOrder
inner join ordersrowsoptions ro on ro.idOrderRow = r.idOrderRow
where r.idProduct = [foo]
My full resultset is blank if no ordersrowsoptions records exist for the given product.
what's the correct syntax to return records even if no records exist at one of the join clauses?
thx
select * from orders o
inner join OrdersRows r on r.idOrder = o.idOrder
left join ordersrowsoptions ro on ro.idOrderRow = r.idOrderRow
where r.idProduct = [foo]
Of course you should not use select * in any query but especially never when doing a join. The repeated fields are just wasting server and network resources.
Since you seem unfamiliar with left joins, you probably also need to understand the concepts in this:
http://wiki.lessthandot.com/index.php/WHERE_conditions_on_a_LEFT_JOIN
LEFT JOIN / RIGHT JOIN.
Edit: yes, the following answer, given earlier, is correct:
select * from orders o
inner join OrdersRows r on r.idOrder = o.idOrder
left join ordersrowsoptions ro on ro.idOrderRow = r.idOrderRow
where r.idProduct = [foo]
LEFT JOIN (or RIGHT JOIN) are probably what you are looking for, depending on which side of the join no rows may appear.
Interesting, do you want to get all orders that have that product in them? The other post is correct that you have to use LEFT or RIGHT OUTER JOINS. But if you want to get entire orders that have that product then you'd need a more complex where clause.