SQL-在where子句中正确连接VS a = b

Question

I was just hoping someone could tell me if there is a difference in efficiency between the 2 queries below.

---

Query 1

SELECT A.CUSTOMER, B.INVOICE FROM CUS A

JOIN INV B ON
A.CUS_ID = B.CUS_ID

WHERE
B.INV_DATE > '2015-09-06'

---

Query 2

SELECT A.CUSTOMER, B.INVOICE FROM CUS A, INV B

WHERE A.CUS_ID = B.CUS_ID AND

B.INV_DATE > '2015-09-06'

---

I am pretty sure query 1 will be more efficient, but I was hoping someone could confirm.

Answer 1

The SELECT * FROM table1, table2, ... syntax is ok for a couple of tables, but it becomes exponentially (not necessarily a mathematically accurate statement) harder and harder to read as the number of tables increases.

The JOIN syntax is harder to write (at the beginning), but it makes it explicit what criteria affects which tables. This makes it much harder to make a mistake.

Also, if all the joins are INNER, then both versions are equivalent. However, the moment you have an OUTER join anywhere in the statement, things get much more complicated and it's virtually guarantee that what you write won't be querying what you think you wrote.

Your performance question has been answered: there is no difference.