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INNER JOIN ON vs WHERE clause
Difference between these two joining table approaches?
There is no difference at all.
Second representation makes query more readable and makes it look very clear as to which join corresponds to which condition.
The queries are logically equivalent. The comma operator is equivalent to an
[INNER] JOIN operator.
The comma is the older style join operator. The JOIN keyword was added later, and is favored because it also allows for OUTER join operations.
It also allows for the join predicates (conditions) to be separated from the
WHERE clause into an
ON clause. That improves (human) readability.
Beside better readability, there is one more case where explicitly joined tables are better instead of comma-separated tables.
let’s see an example:
Create Table table1 ( ID int NOT NULL Identity(1, 1) PRIMARY KEY , Name varchar(50) ) Create Table table2 ( ID int NOT NULL Identity(1, 1) PRIMARY KEY , ID_Table1 INT NOT NULL )
Following query will give me all columns and rows from both tables
SELECT * FROM table1, table2
Following query will give me columns from first table with table alias called ‘table2’
SELECT * FROM table1 table2
If you mistakenly forget comma in comma-separated join, second table automatically convert to table alias for first table. Not in all cases, but there is chances for something like this
The SELECT * FROM table1, table2, etc. is good for a couple of tables, but it becomes exponentially harder as the number of tables increases.
JOIN syntax makes it explicit what criteria affects which tables (giving a condition). Also, the second way is the older standard.
Although, to the database, they end up being the same
Using JOINS makes the code easier to read, since it’s self-explanatory.
In speed there is no difference (I tested it) and the execution plan is the same
If the query optimizer is doing its job right, there should be no difference between those queries. They are just two ways to specify the same desired result.