Question

I have these SQL instructions:

 CREATE TABLE discipline (
       did INT PRIMARY KEY,
       code VARCHAR(127) UNIQUE,
       university VARCHAR(127),
       number_students INT
 ) ENGINE=INNODB; 

 CREATE TABLE precedence(
      type VARCHAR(127),
      basic INT,
      advanced INT,
      PRIMARY KEY (basic, advanced), 
      FOREIGN KEY (basic) REFERENCES discipline(did) ON DELETE CASCADE ON UPDATE CASCADE,
      FOREIGN KEY (advanced) REFERENCES discipline(did)
 )ENGINE=INNODB;

 INSERT INTO discipline VALUES (1,'BD','U1',30);
 INSERT INTO discipline VALUES (2,'IBD','U2',30);
 INSERT INTO discipline VALUES (3,'SIBD','U3',30);
 INSERT INTO precedence VALUES ('optional',1,2);
 INSERT INTO precedence VALUES ('mandatory',2,3);
 UPDATE discipline SET did=did+did WHERE did=2;

I don’t understand why the result of the instruction SELECT SUM (DISTINCT did) is 6 and isn’t 8 (1+4+3). The value 2 (did) with the update instruction shouldn’t be 4? Can you explain this to me? Thanks.

Answer 1

The value 2 (did) with the update instruction shouldn’t be 4?

No, it is 4, and it should.

Because the UPDATE statements in SQL, are All-at-once operations. Meaning that all the expressions that you have, in the same logical phase are evaluated as if at the same point in time. In your example:

UPDATE discipline 
SET did = did + did 
WHERE did=2;

the did will equal to 4 because the expression did = did + did is evaluated wit the value did = 2 at the same time.

SQL Fiddle Demo

This will make your table looks like:

| DID | CODE | UNIVERSITY | NUMBER_STUDENTS |
---------------------------------------------
|   1 |   BD |         U1 |              30 |
|   3 | SIBD |         U3 |              30 |
|   4 |  IBD |         U2 |              30 | <<<<<

---

I don’t understand why the result of the instruction SELECT SUM
(DISTINCT did) is 6 and isn’t 9 (1+4+3).

The values of Did after update will be:

1
3
4

Therefore:

SELECT SUM(DISTINCT did) 
FROM discipline 

Will equal to 8.

SQL Fiddle Demo

---

In your updated example, you won’t be able to UPDATE did, becuase of the foreign key constraint and you will got an exception:

Cannot delete or update a parent row: a foreign key constraint fails
(db_2_65e0c.precedence, CONSTRAINT precedence_ibfk_2 FOREIGN KEY
(advanced) REFERENCES discipline (did))

SQL Fiddle Demo

In this case, you will need to add the ON DELETE CASCADE ON UPDATE CASCADE to the other foreign key constraint:

 FOREIGN KEY (advanced) REFERENCES discipline(did) 

Then the UPDATE:

UPDATE discipline
SET did=did + did
WHERE did=2;

will be the same as explained before, and

SELECT SUM(DISTINCT did) 
FROM discipline 

Will equal to 8.

SQL fiddle demo of the two table(working)