Using count in a Subquery - mysql

Question

Im currently learning about sub querys in mysql and just want to clarify if im on the right track. The below code does bring a result but im unsure if im doing it correctly. The question is
"Find a code and company name of all customers who submitted at least one order in April 1997 using an IN statement ."
SELECT CUSTOMER_CODE, COMPANY_NAME
FROM CUSTOMER
WHERE CUSTOMER_CODE IN (SELECT CUSTOMER_CODE
FROM ORDERS
WHERE ORDER_DATE BETWEEN '1997-04-01' AND '1997-04-30'
GROUP BY CUSTOMER_CODE )
I went through the values that are inserted into the tables and it just doesnt seem to be correct even though a result is produced.
(Side note, is using a EXIST clause the same as a IN clause, but instead of WHERE CUSTOMER_CODE IN it is WHERE EXISTS ?
I tried this
SELECT CUSTOMER_CODE, COMPANY_NAME
FROM CUSTOMER
WHERE NOT EXISTS (SELECT CUSTOMER_CODE
FROM ORDERS
WHERE ORDER_DATE BETWEEN '1997-04-01' AND '1997-04-30'
GROUP BY CUSTOMER_CODE );
these are the two talbes im using
CREATE TABLE CUSTOMER
(
CUSTOMER_CODE VARCHAR(5) NOT NULL,
COMPANY_NAME VARCHAR(40) NOT NULL,
CONTACT_NAME VARCHAR(30),
CONTACT_TITLE VARCHAR(30),
ADDRESS VARCHAR(60),
CITY VARCHAR(15),
REGION VARCHAR(15),
POSTAL_CODE VARCHAR(10),
COUNTRY VARCHAR(15),
PHONE VARCHAR(24),
FAX VARCHAR(24),
CONSTRAINT PK_CUSTOMER PRIMARY KEY (CUSTOMER_CODE)
);
CREATE TABLE ORDERS
(
ORDER_ID DECIMAL(9) NOT NULL,
CUSTOMER_CODE VARCHAR(5) NOT NULL,
EMPLOYEE_ID DECIMAL(9) NOT NULL,
ORDER_DATE DATE NOT NULL,
REQUIRED_DATE DATE,
SHIPPED_DATE DATE,
SHIP_VIA VARCHAR(40),
FREIGHT DECIMAL(10,2) DEFAULT 0,
SHIP_NAME VARCHAR(40),
SHIP_ADDRESS VARCHAR(60),
SHIP_CITY VARCHAR(15),
SHIP_REGION VARCHAR(15),
SHIP_POSTAL_CODE VARCHAR(10),
SHIP_COUNTRY VARCHAR(15),
CONSTRAINT PK_ORDERS PRIMARY KEY (ORDER_ID),
CONSTRAINT FK_CUSTOMER_CODE FOREIGN KEY (CUSTOMER_CODE) REFERENCES CUSTOMER(CUSTOMER_CODE),
CONSTRAINT FK_EMPLOYEE_ID FOREIGN KEY (EMPLOYEE_ID) REFERENCES EMPLOYEE(EMPLOYEE_ID),
CONSTRAINT FK_SHIP_VIA FOREIGN KEY (SHIP_VIA) REFERENCES SHIPPER(COMPANY_NAME)
);
but i recieve an empty set
Thankyou all for your time

Answer 1

instead of a IN clause you could use a INNER JOIN based on the same subquery ..
usually this perform better that a In clause
SELECT CUSTOMER_CODE, COMPANY_NAME
FROM CUSTOMER
INNER JOIN (
SELECT DISTINCT CUSTOMER_CODE
FROM ORDERS
WHERE ORDER_DATE BETWEEN '1997-04-01' AND '1997-04-30'
) t on t.CUSTOMER_CODE = CUSTOMER.CUSTOMER_CODE
You should not use group by for when you don't use aggregation function (this don't work by default in most recent version of mysql)
use DISTINCT if you need only distinct result

Answer 2

You must use EXISTS instead of NOT EXISTS:
SELECT c.CUSTOMER_CODE, c.COMPANY_NAME
FROM CUSTOMER c
WHERE EXISTS (
SELECT CUSTOMER_CODE FROM ORDERS
WHERE
CUSTOMER_CODE = c.CUSTOMER_CODE
AND
ORDER_DATE BETWEEN '1997-04-01' AND '1997-04-30'
);
You can also check about the ORDER_DATE with this:
YEAR(ORDER_DATE) = 1997 AND MONTH(ORDER_DATE) = 4
You can also do it with IN:
SELECT CUSTOMER_CODE, COMPANY_NAME
FROM CUSTOMER
WHERE CUSTOMER_CODE IN (
SELECT CUSTOMER_CODE FROM ORDERS
WHERE YEAR(ORDER_DATE) = 1997 AND MONTH(ORDER_DATE) = 4
);
If you want to find the total number of all customers who submitted no orders:
SELECT COUNT(DISTINCT c.CUSTOMER_CODE) AS COUNTER
FROM CUSTOMER c
WHERE NOT EXISTS (
SELECT CUSTOMER_CODE FROM ORDERS
WHERE CUSTOMER_CODE = c.CUSTOMER_CODE
)