Error 1242: Sub query returns more than 1 row and problem with repeating an if statement for all rows

Question

So I've just started learning MySQL with a few exercises and I'm currently stuck at these 2 problems. Since this is technically an exercise I'd really appreciate hints instead of full solutions. Here are the 2 tables first:

CREATE TABLE LINE (
INV_NUMBER int,
LINE_NUMBER int,
P_CODE varchar(10),
LINE_UNITS float(8),
LINE_PRICE float(8),
LINE_TOTAL float(8)
);
INSERT INTO LINE VALUES('1001','1','13-Q2/P2','1','14.99','14.99');
INSERT INTO LINE VALUES('1001','2','23109-HB','1','9.95','9.95');
INSERT INTO LINE VALUES('1002','1','54778-2T','2','4.99','9.98');
INSERT INTO LINE VALUES('1003','1','2238/QPD','1','38.95','38.95');
INSERT INTO LINE VALUES('1003','2','1546-QQ2','1','39.95','39.95');
INSERT INTO LINE VALUES('1003','3','13-Q2/P2','5','14.99','74.95');
INSERT INTO LINE VALUES('1004','1','54778-2T','3','4.99','14.97');
INSERT INTO LINE VALUES('1004','2','23109-HB','2','9.95','19.90');
INSERT INTO LINE VALUES('1005','1','PVC23DRT','12','5.87','70.44');
INSERT INTO LINE VALUES('1006','1','SM-18277','3','6.99','20.97');
INSERT INTO LINE VALUES('1006','2','2232/QTY','1','109.92','109.92');
INSERT INTO LINE VALUES('1006','3','23109-HB','1','9.95','9.95');
INSERT INTO LINE VALUES('1006','4','89-WRE-Q','1','256.99','256.99');
INSERT INTO LINE VALUES('1007','1','13-Q2/P2','2','14.99','29.98');
INSERT INTO LINE VALUES('1007','2','54778-2T','1','4.99','4.99');
INSERT INTO LINE VALUES('1008','1','PVC23DRT','5','5.87','29.35');
INSERT INTO LINE VALUES('1008','2','WR3/TT3','3','119.95','359.85');
INSERT INTO LINE VALUES('1008','3','23109-HB','1','9.95','9.95');

CREATE TABLE PRODUCT (
P_CODE varchar(10),
P_DESCRIPT varchar(35),
P_INDATE date,
P_QOH int,
P_MIN int,
P_PRICE float(8),
P_DISCOUNT float(8),
V_CODE int
);
INSERT INTO PRODUCT VALUES('11QER/31','Power painter, 15 psi., 3- 
nozzle','2015-11-03','8','5','109.99','0','25595');
INSERT INTO PRODUCT VALUES('13-Q2/P2','7.25-in. pwr. saw blade','2015-12- 
13','32','15','14.99','0.05',NULL);
INSERT INTO PRODUCT VALUES('14-Q1/L3','9.00-in. pwr. saw blade','2015-11- 
13','18','12','17.49','0','21344');
INSERT INTO PRODUCT VALUES('1546-QQ2','Hrd. cloth, 1/4-in., 2x50','2016-01- 
15','15','8','39.95','0','23119');
INSERT INTO PRODUCT VALUES('1558-QW1','Hrd. cloth, 1/2-in., 3x50','2016-01- 
15','23','5','43.99','0','23119');
INSERT INTO PRODUCT VALUES('2232/QTY','B&D jigsaw, 12-in. blade','2015-12- 
30','8','5','109.92','0.05','24288');
INSERT INTO PRODUCT VALUES('2232/QWE','B&D jigsaw, 8-in. blade','2015-12- 
24','6','5','99.87','0.05','24288');
INSERT INTO PRODUCT VALUES('2238/QPD','B&D cordless drill, 1/2-in.','2016- 
01-20','12','5','38.95','0.05','25595');
INSERT INTO PRODUCT VALUES('23109-HB','Claw hammer','2016-01- 
12','23','10','9.95','0.1','21225');
INSERT INTO PRODUCT VALUES('23114-AA','Sledge hammer, 12 lb.','2016-01- 
2','8','5','14.40','0.05',NULL);
INSERT INTO PRODUCT VALUES('54778-2T','Rat-tail file, 1/8-in. fine','2015- 
12-15','43','20','4.99','0','21344');
INSERT INTO PRODUCT VALUES('89-WRE-Q','Hicut chain saw, 16 in.','2016-02- 
17','11','5','256.99','0.05','24288');
INSERT INTO PRODUCT VALUES('PVC23DRT','PVC pipe, 3.5-in., 8-ft','2016-02- 
27','188','75','5.87','0','24004');
INSERT INTO PRODUCT VALUES('SM-18277','1.25-in. metal screw, ''25','2016-03- 
01','172','75','6.99','0','21225');
INSERT INTO PRODUCT VALUES('SW-23116','2.5-in. wd. screw, 50','2016-02- 
14','237','100','8.45','0','21231');
INSERT INTO PRODUCT VALUES('WR3/TT3','Steel matting, 4''x8''x1/6", .5" 
mesh','2016-01-27','18','5','119.95','0.1','25595');

PKs are line.INV_NUMBER, line.LINE_NUMBER, product.P_CODE and FKs are line.P_CODE and line.V_CODE

So the first question is to list all product sales that are greater than the average units sold for that product and add a correlated in-line sub-query to list the average units sold per product. If I understood correctly this means that I need to sum the line.LINE_UNITS group by P_CODE then compare it to the average units sold but I keep getting the same error that says sub query returns more than 1 row. Here's my code:

select  P_CODE,
        LINE_UNITS, 
        ( SELECT  AVG(line.LINE_UNITS) ) as ‘Unit_Average’
    from  line
    where  
        ( SELECT  sum(line.LINE_UNITS)
            from  line
            group by  P_CODE ) >
        ( SELECT  AVG(line.LINE_UNITS) );

I think I need to do a join instead and I'd really appreciate some hints.

For the second question I am supposed to list the difference between each product's prices and the average product price. I managed to type up a few lines but I couldn't get it to execute for all rows of the product table. I'm thinking of using over() but im having quite a lot of syntax errors. Here's my code:

select  P_CODE, P_PRICE, 
        ( SELECT  avg(product.P_PRICE) ) as 'Average',
        if(product.P_PRICE <= 
              ( SELECT  avg(product.P_PRICE) ), 
              ( SELECT  avg(product.P_PRICE) ) - product.P_PRICE,
              product.P_PRICE - ( SELECT  avg(product.P_PRICE) )
        ) as 'Difference'
    from  product;

Thanks guys!

Answer 1

For your first question, I would start by figuring out what the average for each product are:

select p_code, avg(line_units) as line_units 
from LINE 
group by p_code;

SQL is closed in the sense that every result of a query is a table, so you can use the result just like an ordinary table. In other words, we can join this query with the original table:

select l.* 
from LINE as l 
join (
    select p_code, avg(line_units) as line_units 
    from LINE 
    group by p_code
) as avg_l 
    on l.p_code = avg_l.p_code 
   and l.line_units > avg_l.line_units;

Latest version of MySQL finally implements window functions from SQL99 which makes it a bit easier (untested)

select INV_NUMBER, LINE_NUMBER, P_CODE, LINE_UNITS
     , LINE_PRICE, LINE_TOTAL
from (
    select INV_NUMBER, LINE_NUMBER, P_CODE, LINE_UNITS
         , LINE_PRICE, LINE_TOTAL
         , AVG(LINE_UNITS) OVER (PARTITION BY P_CODE) AS AVG_LINE_UNITS
    from line
) AS T
WHERE LINE_UNITS > AVG_LINE_UNITS

I believe you should be able to solve question 2 using any of the above techniques. Let me know if you encounter any problems

Answer 2

Hints:

• Think about what product.P_PRICE - ( SELECT avg(product.P_PRICE) ) gives you depending on which is larger.

• Don't use FLOAT for money, nor for integral values ("units").

• Not all quotes are equal.

• You can't use aggregate functions in WHERE, but you can in HAVING.

• Prefixing columns with the name of the table is clutter. If you are using a JOIN, you can (and should) use aliases for clarity: SELECT p.price, ... JOIN Products AS p

• A SELECT usually needs a FROM.

• Think about what you can do with SELECT @avg_price := avg(p.PRICE) FROM products AS p as a separate statement.