I am working on a bunch of orders and I have an order date and the date that the order was "purchased". (The day we processed the order). If we never processed the order because the customer never followed through, there is a blank spot left in the purchased date resulting in a null when I run my query. I attempted to use IS NOT NULL to filter it out in the where statement but nothing happened. Any thoughts?

WHERE
DATE(a.ORDER_DATE) BETWEEN DATE(:startDate) AND DATE(:endDate)
AND
a.PURCH_ORDER_DATE IS NOT NULL;

EDIT: For all of those who are wondering, I figured out how to sort out the null values.

SELECT DISTINCT
a.ORDER_NUMBER
,a.SALES_CATEGORY
,a.ORDER_DATE
,a.PURCH_ORDER_DATE
,CASE
    WHEN DATEDIFF(a.ORDER_DATE,a.PURCH_ORDER_DATE) >= 0 THEN DATEDIFF(a.ORDER_DATE,a.PURCH_ORDER_DATE)
    ELSE "null"
 END as Lag
FROM
SOBOOK a

WHERE
DATE(a.ORDER_DATE) BETWEEN DATE(:startDate) AND DATE(:endDate)
AND
ceil(DATEDIFF(a.ORDER_DATE,a.PURCH_ORDER_DATE)) = DATEDIFF(a.ORDER_DATE,a.PURCH_ORDER_DATE)

ORDER BY
 ORDER_NUMBER
,Lag

Hi! Me and my sibling-in-law are just beggining to learn SQL and are about to get in a boot camp that gives you an introductory "exam". We failed it the first time, but weren't told why. This Exam willl change, so we're not looking to have our homework done so to say, we just want to understand what we did wrong in the first try.

And after watching a lot of videos and trying different solutions, we're a bit confused about this schema:

What we can't get a grasp on is what's the use of NOT NULL here? Like, how should we add that to our querys?

We're also a bit lost when it comes to item 10, how should we use "join" here?

Thank you in advance, we're doing our best!

I'll translate all the questions so that there's some context:

The first point was:

"Write an SQL query to show all the products in the table "Productos" with a price higher to $50."
Our answer was:

Select * from productos where Price > 50

Second point was:
"Write an SQL query to obtain the total amount of orders (pedidos) made by an specific client according to his ID"
Our answer was:

Select cliente_ID, count(*) as Pedidos_count
from Pedidos
where cliente_ID= ‘NOT NULL’
group by cliente_ID

Third point was:
"Write an SQL query to update the price of a product on the table "Productos""
Our answer was:

Update productos set price = ‘Float’
where nombre = ‘Varchar’

Fourth point was:
"Write an SQL query to show the names of the products together with their corresponding categories."
Our answer was:

Select nombre_varchar, categoria_varchar from productos

Fifth point was:
"Write an SQL query to delete all the orders that have an amount lesser than 5."
Our answer was:

Delete from pedidos where quantity < 5

Sixth point was:
"Write an SQL query to calculate the total price of the orders made."
Our answer was:

Select SUM (total_precio) as "total_pedidos_precio"
From Pedidos

Seventh point was:
"Write an SQL query to show the names of the products in ascendant alphabetical order."
Our answer was:

select * from productos
Order by nombre asc

Eighth point was:
"Write an SQL query to show the orders made in a specific date." (fecha means date).
Our answer was:

select * from Pedidos where date (fecha_pedido) = NOT NULL

Ninth point was:
"Write an SQL query to obtain the average of the prices of all the products."
Our answer was:

Select AVG (precio) from Productos

Tenth point was:
"Write an SQL query to show the products together with the total amount of orders made for each one."
We weren't sure about this one, we think we have to use the join clause, but we couldn't agree on how to.

Eleventh point was:
"What's the correct syntax to insert a new record in the table "Usuarios" (Users)"
a) INSERT INTO Usuarios (Nombre, Apellido) VALUES ('John', 'Doe'); (Picked this one)
b) INSERT Usuarios (Nombre, Apellido) VALUES ('John', 'Doe');
c) INSERT VALUES ('John', 'Doe') INTO Usuarios;
d) INSERT INTO Usuarios VALUES ('John', 'Doe');

Twelfth point was:
"What's the function used to obtain the total amount of records in a table?"
a) COUNT() (Picked this one)
b) SUM()
c) AVG()
d) MAX()

Thirteenth point was:
"What's the clause used to filter results in a SELECT query?"
a) WHERE (Picked this one)
b) FROM
c) ORDER BY
d) GROUP BY

Fourteenth point was:
"What's the operator used to combine conditions in a WHERE clause?"
a) OR
b) AND (Picked this one)
c) NOT
d) XOR

Fifteenth point was:
"What's the SQL query to delete an existing table?"
a) DELETE TABLE name_table; (Picked this one)
b) DROP name_table;
c) REMOVE name_table;
d) ERASE name_table;