Why can I insert a number into a VARCHAR2 column? - sql

Question

I have an odd problem with a table, I'm inserting rows into tables, and now I am trying to test it, on one of my tests it allows an entry that it shouldn't.
My table definition:
CREATE TABLE ITEMS(
ITEM_NUMBER VARCHAR2(3)
CONSTRAINT ITEMS_ITEM_NUMBERf_PK PRIMARY KEY,
ITEM_DESCRIPTION VARCHAR2(30),
ITEM_SIZE VARCHAR2(2),
ITEM_COST NUMBER(30,0),
ITEM_QTY NUMBER(2,0),
ORDER_NUMBER VARCHAR2(4));
When I try and test the ITEM_SIZE column by putting in a number it just inserts it.
INSERT INTO ITEMS VALUES ('I32', 'Leather blazer', 'L', 14.00, 1, 'O114');
INSERT INTO ITEMS VALUES ('I33', 'Fleece vest', 'L', 28.00, 1, 'O128');
INSERT INTO ITEMS VALUES ('I34', 'Khaki pants', 'S', 22.00, 3, 'O122');
INSERT INTO ITEMS VALUES ('I35', 'Muscle Tee', 'M', 12.99, 1, 'O122');
INSERT INTO ITEMS VALUES ('I36', 'Trench Coat', 'M', 03.00, 1, 'O133');
INSERT INTO ITEMS VALUES ('I37', 'Crewneck sweater', 'XL', 07.00, 1, 'O107');
INSERT INTO ITEMS VALUES ('I38', 'Varsity Sweater', 'M', 08.99, 1, 'O108');
INSERT INTO ITEMS VALUES ('I39', 'CROOK', 'M', 12.00, 1, 'O112');
//--the tester--//
INSERT INTO ITEMS VALUES ('I49', 'bROOK', 10, 12.00, 1, 'O112');
In the statement above I insert the number 10 into ITEM_SIZE VARCHAR2(2). How can I avoid that?

Answer 1

As #kevinsky said, Oracle is doing implicit data conversion from number to string while you insert.
If you want to restrict the values that can be put into a column then you can add a check constraint:
alter table items add constraint items_size_chk
check (item_size in ('XXS','XS','S','M','L','XL','XXL','XXXL'));
Table ITEMS altered.
INSERT INTO ITEMS VALUES ('I49', 'bROOK', 10, 12.00, 1, 'O112');
SQL Error: ORA-02290: check constraint (SCHEMA.ITEMS_SIZE_CHK) violated
02290. 00000 - "check constraint (%s.%s) violated"
*Cause: The values being inserted do not satisfy the named check
If you want different restrictions you can change the way the check is done; for example you could have a regular expression check to exclude digits but allow any characters. In this case it seems more likely you want a list of specific values to be allowed though.
Note though that when a new valid size is added, you will have to modify the check constraint; and any of those size values will be allowed for any item.
If the valid sizes are coming from another table then you can use a foreign key constraint instead. You probably really want a list of sizes (or other attributes) that are valid for each item, so your data model might need some fleshing out.

Answer 2

Oracle is implicitly converting the number 10 into the varchar2 string '10'.
and here is another implicit conversion
SELECT item_size + 1 FROM items;
Result in Oracle 11g is
11
I'm not sure whether this was intended to be helpful when the parser was being built but it does cause a lot of confusion. However Oracle will not convert an alphabetical character into a number implicitly. It will fail.