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mysqlwhere-clause
Suvarna Pattayil
Suvarna Pattayil

Reputation: 5239

How does WHERE clause work (interchanging LHS and RHS)

I tried the following query in MySQL,

select first_name from my_contacts WHERE 'Blunt' = last_name;

and it works fine.

Just like,

select first_name from my_contacts WHERE last_name = 'Blunt';

So how does WHERE clause work . I assumed when it encounters a WHERE it expects it to be followed by a column name and RHS of = with a value (In this case a String).

But it seems even interchanging LHS and RHS is fine. Is it similar to if(a == 1) or if(1 == a) in C or C++ (Doesn't matter which is RHS or LHS)?

Note: If there are links that can help me in this matter pls do include them

Upvotes: 2

Views: 738

Answers (2)

jpmc26
jpmc26

Reputation: 29934

Much like a compiler does with C source code, MySQL (or any database) parses the query text and uses the syntax tree to create something it can execute, a query plan, namely. MySQL is figuring out what the query means piece by piece, and it uses a grammar like any programming language to try to figure out what each part is. As Borniet notes, we are allowed to put even a constant expression inside a where clause. We could also compare two columns for equality. This is because MySQL allows WHERE to use any expression with a result that can be coerced into to a boolean value.

Upvotes: 2

Borniet
Borniet

Reputation: 3546

It's indeed very much comparable to an IF statement:

WHERE (statement that needs to be true) AND (a = 1) OR (B = 2)

So you could also do:

WHERE 1

Upvotes: 1

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