Home » Mysql » What's the difference between comma separated joins and join on syntax in MySQL? [duplicate]

What's the difference between comma separated joins and join on syntax in MySQL? [duplicate]

Posted by: admin November 1, 2017 Leave a comment

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Answers:

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.

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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.

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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

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The SELECT * FROM table1, table2, etc. is good for a couple of tables, but it becomes exponentially harder as the number of tables increases.

The 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

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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.