MySQL Join Subquery – Multiple Subqueries in Columns with Joins to Return Multiple Rows

join;MySQLsubquery

I have a table which stores id(primary key) of users and not their names (Names are stored in another master table) and also the the department id of user and not the name of the departments and there is one more column joining year. Now I want to return the data like Name(actual name of user), Department(actual name of the department) and year by using subqueries. I tried but I am not getting any results.

main table
--------------------------
| user | department| year|
--------------------------
| 1    |  1        | 1992|
--------------------------
|  2   | 12        | 2007|
--------------------------
users table
--------------------
| pkey | name      | 
--------------------
| 1    |  someone  | 
--------------------
|  2   |  someName | 
--------------------
department table
--------------------
| pkey | name      | 
--------------------
| 1    |  Dep1     | 
--------------------
|  12  |  Dep2     | 
--------------------
expected result from query for main table with subquery for user and department by doing inner joins
--------------------------
| user     | department| year|
--------------------------
| someone  |  Dep1     | 1992|
--------------------------
| someName |  Dep2     | 2007|
--------------------------

Best Answer

why you need use subqueries ?

You can use joins for get data

SELECT u.name AS user, d.name AS departement, m.year
FROM main AS m
INNER JOIN department AS d ON m.departement_id = d.id
INNER JOIN users AS u ON m.user_id = u.id
;

I created a sample fidle http://sqlfiddle.com/#!9/74a4e2/1

Related Solutions

Mysql – Subqueries run very fast individually, but when joined are very slow

You don't need all the derived tables. You are joining the basic (product) too many times. You can write the query joining it only once.

Compound indices are a must for EAV designs. Try adding an index on (attribute_id, product_id, value) and then the query:

SELECT t0.id, 
       t1.`value` AS length, 
       t2.`value` AS height, 
       t3.`value` AS family
FROM
  products t0

INNER JOIN 
  product_eav_decimal t1
    ON  t1.product_id = t0.id  
    AND t1.attribute_id = 91
    AND t1.`value` BETWEEN 15 AND 35

LEFT JOIN
  product_eav_decimal t2
    ON  t2.product_id = t0.id  
    AND t2.attribute_id = 80  
-- 
-- 
--

LEFT JOIN                              -- LEFT or INNER join
  product_eav_decimal t6
    ON  t6.product_id = t0.id  
 -- AND t6.attribute_id = 

ORDER BY t0.id ASC ;

SQL – How to Use Grouped Data Multiple Times in a Query

The following will get you the assignment answer. There are many ways to solve this, and it is not necessarily bad to group several times in sub-queries. Most RDBMS engines will be able to optimize your query (some) and performance is likely not an issue for this particular question.

SELECT TOP (1)
     Department
    ,SumSalary
    ,AvgSalary
FROM
(
SELECT
     Department
     ,SumSalary
    ,AVG(SumSalary) OVER(PARTITION BY NULL) AS AvgSalary
FROM
(
SELECT
     Department
    ,SUM(Salary) AS SumSalary
FROM DIM.Property
GROUP BY Department
) SubQuery
) x
WHERE SumSalary > AvgSalary
ORDER BY SumSalary DESC

I'd suggest that you read up on Window Functions so that you can fully explain any work submitted (there are plenty of sites and technical documentation for each DB and ANSI SQL).

Also note that a TOP (1) gets just the single highest paying department that is above the average, since you may have many that meet this criteria. To ensure you get the highest paying the ORDER BY guarantees the highest paying is on top. You still have the possibility of two departments paying the same amount though in which case you have a non-deterministic query (not guaranteed to get the same department every time). You need a tie-breaker unless you accept this potential for random results or it is guaranteed not to happen by some other business process or rule (yeah right!).

Related Question