ERROR: missing FROM-clause entry for table when running function - postgresql

Question

I want to create a trigger and a function that do the following: Each time a new row is inserted in table SalesOrderDetail the function finds the corresponding CustomerID from table SalesOrderHeader and then it adds +1 to the corresponding number_of_sales in the Customer table.
SalesOrderDetail
+---------+-------------------------+
| SalesOrderID | SalesOrderDetailID |
+---------+-------------------------+
| value1 | value4 |
| value1 | value5 |
| value2 | value6 |
| value3 | value7 |
| value3 | value8 |
| value4 | value9 |
+---------+-------------------------+
SalesOrderHeader
+---------+-----------------+
| SalesOrderID | CustomerID |
+---------+-----------------+
| value1 | value10 |
| value2 | value11 |
| value3 | value12 |
| value4 | value13 |
+---------+-----------------+
Customer
+---------+--------------------+
| CustomerID | Number_of_sales |
+---------+--------------------+
| value10 | value14 |
| value11 | value15 |
| value12 | value16 |
| value13 | value17 |
+---------+--------------------+
Code is as follows:
CREATE OR REPLACE FUNCTION new_order_detail()
RETURNS trigger AS
$BODY$
BEGIN
DROP TABLE IF EXISTS CustomerInfo;
CREATE TEMP TABLE CustomerInfo AS
SELECT* FROM(SELECT CustomerID FROM(
SELECT * from SalesOrderHeader
WHERE SalesOrderHeader.SalesOrderID = (SELECT SalesOrderID FROM SalesOrderDetail ORDER BY SalesOrderID DESC limit 1))AS Last_Entry) AS Common_Element;
IF CustomerInfo.CustomerID = Customer.CustomerID THEN
UPDATE Customer
SET number_of_items = number_of_items + 1;
END IF;
END;
$BODY$ LANGUAGE plpgsql;
DROP TRIGGER IF EXISTS new_order ON SalesOrderDetail;
CREATE TRIGGER new_order
AFTER INSERT OR UPDATE ON SalesOrderDetail
FOR EACH ROW EXECUTE PROCEDURE new_order_detail();
When I insert something into the SalesOrderDetail table I get the following error:
PL/pgSQL function new_order_detail() line 3 at SQL statement ERROR:
missing FROM-clause entry for table "customerinfo" LINE 1: SELECT
CustomerInfo.CustomerID = Customer.CustomerID
^ QUERY: SELECT CustomerInfo.CustomerID = Customer.CustomerID CONTEXT: PL/pgSQL function new_order_detail()
line 12 at IF
********** Error **********
ERROR: missing FROM-clause entry for table "customerinfo" SQL state:
42P01 Context: PL/pgSQL function new_order_detail() line 12 at IF
What I am doing wrong? Sorry for the poor explanation English is not my native language.

Answer 1

Firstly you need to take care about the case-sensitive pgsql. If you do not explicitly use "", it converts all to lower case.
Next, your procedure should be somewhat like this:
select CustomerID from SalesOrderHeader where SalesOrderDetail.SalesOrderID = SalesOrderHeader.SalesOrderID into custID;
UPDATE Customer SET number_of_items = number_of_items + 1 where CustomerID = custID ;