I have to tables who looks like this
Table1
User_ID(int)|comment(text)|gender(int)
so it could be like 1|bla bla|1
Table2
ID(int)|Username(text)
Now I want to build a query like
SELECT Table1.User_ID,Table1.comment,Table1.gender FROM Table1 INNER JOIN Table2 ON Table1.User_ID=Table2.ID SELECT Username
Is something like that working? I hope my query is not that bad to understand. If thats working in one way or another, is it also possible to make some more joins?
First time I have to work with joins. I'm a bit irritated, most examples are a bit abstract,..
Just list every columns that you want to select after the SELECT statement, even if they come from joined tables.
SELECT Table1.User_ID, Table1.comment, Table1.gender, Table2.Username
FROM Table1 INNER JOIN Table2
ON Table1.User_ID = Table2.ID
You could have as many joins as you want:
SELECT Table1.User_ID, Table1.comment, Table1.gender, Table2.Username
FROM Table1
INNER JOIN Table2
ON Table1.User_ID = Table2.ID
INNER JOIN Gender
ON Table1.gender = Gender.id
Yes and yes. You can access table 2 fields in select statement also.
Related
This question is different than the ones I have seen already answered. Is it possible to left join using multiple 'FROM' tables? When I try it (with other code, but the principle is the same) I get the error 'Unknown column 'table1.otherId' in 'on clause'. Any help would be greatly appreciated.
Something like this:
SELECT *
FROM table1, table2
LEFT JOIN other_table
ON other_table.id = table2.id
AND other_table.otherId = table1.otherId
Try like this:-
SELECT *
FROM table1
LEFT JOIN other_table
ON table1.id=other_table.id LEFT JOIN table2 ON
other_table.otherId = table2.id
It depends what you want to do. You seem to want a Cartesian product of the first two tables with lookups on the third:
SELECT *
FROM table1 CROSS JOIN
table2 LEFT JOIN
other_table
ON other_table.id = table2.id AND
other_table.otherId = table1.otherId;
Commas -- which should just be banned permanently from FROM clauses -- are similar to CROSS JOINs. However, the parsing of the SQL statement is different. A comma prevents the tables before it from being used in ON clauses after it; that is the source of your error.
Answer is "No"
But you can do it this way:
SELECT * FROM table2, other_table ON other_table.id = table2.id
LEFT JOIN table1 ON other_table.otherId = table1.otherId
I am trying to join two table using left join, that is table1 left join table2.
I would only like part of the rows from A to be joined with B. Is it recommended that i use a sub query to filter rows from table1 or avoid them in where clause to improve my query performance?
select t1.a
,t1.b
,t2.c
from (select *
from table1
where a='x'
) t1 LEFT JOIN table2 t2 on t1.d=t2.d
or
select t1.a
,t1.b
,t2.c
from table1 t1 LEFT JOIN table2 t2 on t1.d=t2.d
where t1.a='x'
Check the query plan but I doubt it would make any difference.
It very depends on the structure and content of your database. The best way is to look into the query plan and compare it for both versions of your query.
You can find this documentation useful: MySQL Query Execution Plan
I have stumped on this as I am a total beginner in MySql.
Here is a the basic of how the two tables are formed
Table 1
id,product_id, product_name
Table 2
id,product_id,active
Now i know how to do a select statement to query the results from one table but when I have to involve two, I am lost. Not sure if I have to use inner join, left join etc.
So how can I return the results of the product_id from table 1 only if in table 2 is active?
You could use JOIN (as Fosco pointed out), but you can do the same thing in the WHERE clause. I've noticed that it's a bit more intuitive method than JOIN especially for someone who's learning SQL. This query joins the two tables according to product_id and returns those products that are active. I'm assuming "active" is boolean type.
SELECT t1.*
FROM Table1 t1, Table2 t2
WHERE t1.product_id = t2.product_id AND t2.active = TRUE
W3Schools has a good basic level tutorial of different kinds of JOINs. See INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN.
It's pretty simple to join two tables:
select t1.*
from Table1 t1
join Table2 t2 on t1.product_id = t2.product_id
where t2.active = 'Y'
I'm looking for a query to select rows from two different tables, keeping the column names the same (I did find one result here for selecting from two different tables, but it merged the column names to have an easier query). I need to keep the original column names, but have two different tables existing within the new, larger table. There are no overlapping columns between the two tables.
A picture, to visualise:
So, how can I do this? I know the query will probably be quite convoluted, but anything half-decent is probably going to be better than my current attempt:
SELECT t1.* , t2.*
FROM table1 t1 RIGHT OUTER JOIN table2 t2
ON r.someColumn1 = rc.someColumn2
UNION
SELECT t1.* , t2.*
FROM table1 t1 LEFT OUTER JOIN table2 t2
ON r.someColumn1 = rc.someColumn2
This does work, but only as long as there are no cases where someColumn1 = someColumn2 - which can happen quite easily, of course.
Any help is appreciated, and I apologise for what is probably a very silly question to which the smart answer is "don't do it, you fool!".
You can set your join criterion to never match:
SELECT t1.* , t2.*
FROM table1 t1 RIGHT OUTER JOIN table2 t2
ON 1 = 0
UNION
SELECT t1.* , t2.*
FROM table1 t1 LEFT OUTER JOIN table2 t2
ON 1 = 0
I don't have MySQL to test, but it works in SQL Server.
Edit: my first answer was wrong:
select * from Events
left join GroupList on ID=null
union
select Events.*,GroupList.* from GroupList
left join Events on GID=null
In the above GID and ID are keyfields in the tables.
If I want to perform joins on 3 or more tables, what is the best syntax?
This is my attempt:
Select *
from table1
inner join table2 using id1, table2
inner join table3 using id2, table3
inner join table4 using id4
where table2.column1="something"
and table3.column4="something_else";
does that look right? The things I'm not sure about are
1) do I need to seperate the joins with a comma
2) am I right to make all my joins first and then put my conditions after that?
3) would I be better to use sub-queries and if so what is the corect syntax
Thanks for any advice!
Try to avoid using * where possible.
Specify exactly the data you want returned.
Format your queries using a standard style.
Pick a style you like and keep to it.
You will thank yourself later when your queries get more complex.
Most optimizers will recognize when a condition in a WHERE clause implies an INNER JOIN, but there's no reason not to code that explicitly; if nothing else it keeps your WHERE clause manageable.
Be explicit about what columns you join on. Be explicit about the type of join you're using. USING seems like a shortcut that could get you into trouble.
MySQL has traditionally not handled subqueries as well as could be hoped. That may be changing in newer versions, but there are other ways to get your data without relying on them.
Welcome to the wonderful world of relational databases!
select t1.*
, t2.*
, t3.*
, t4.*
from table1 t1
inner join table2 t2
on t1.id = t2.t1_id
and
t2.column1 = "something"
inner join table3 t3
on t2.id = t3.t2_id
and
t3.column4 = "something_else"
inner join table4 t4
on t3.id = t4.t3_id;
1) do I need to seperate the joins with a comma
No
2) am I right to make all my joins first and then put my conditions after that?
Yes
3) would I be better to use sub-queries and if so what is the corect syntax
No. Joining tables is the preferred and correct way.
Joins are not separated by a comma
ANSI syntax puts the joins first then where condition
e.g. SELECT * FROM table1 INNER JOIN table2 ON table1.id = table2.id WHERE table2.column1='Something'
I'm not 100% sure what you are trying to achieve. But it looks like you do not need to use subqueries.
A subquery would be executed for every row, it sounds as though you could run a more efficient query just using inner joins.
Hope that helps. If you can elaborate a little I will provide more explanation.
Given your requirement that table2 gets joind on the id1-columns in table1 and table2, table3 gets joind on the id2-columns in table2 and table3 and table4 gets joind on the id3-columns in table3 and table4 you'll have to do:
SELECT *
FROM table1
INNER JOIN table2 ON table2.id1 = table1.id1
INNER JOIN table3 ON table3.id2 = table2.id2
INNER JOIN table4 ON table4.id3 = table3.id3
WHERE table2.column1 = "something"
AND table3.column4 = "something_else"
I think this statement is much more clearer on what is exactly joined in which way - compared to the USING-statement.
Remove the comma's and the duplicate table names, like:
Select *
from table1
inner join table2 using id1
inner join table3 using id2
inner join table4 using id4
where table2.column1="something"
and table3.column4="something_else"
If id4 has a different name in table1, explicitly name the join condition, for example:
inner join table4 on table4.id = table1.table4i
You may be able to use natural join which joins on field names common to the tables you want to join as follows.
SELECT *
FROM table1
NATURAL JOIN table2
NATURAL JOIN table3
NATURAL table4
WHERE table2.column1 = "something"
AND table3.column4 = "something_else"