i'm learning Mysql
but i felt confused after the examples below :
select * from `users` where username = 'admin' or true;
here it returned all the rows in users table !
(username = 'admin' or true ) should be true ? so where true
but in this example :
select * from `users` where username = 'admin' and true;
it returned one row (where username = 'admin')
but (username = 'admin' and true) should be true too !
so what's is the difference?
-- this is always true
WHERE 1
-- this is only true for rows where the username is admin
WHERE username = 'admin'
Now check this truth table :
x y | x and y | x or y
-----------------------
F F | F | F
F T | F | T
T F | F | T
T T | T | T
https://en.wikipedia.org/wiki/Boolean_algebra
If you take
x
for WHERE username = 'admin'
and y
for WHERE 1
you should understand the results.
you should read it differently.
the clause checks to see if username equals 'admin'.
read it as follows
(username = 'admin') and (1)
and for the or
(username = 'admin') or (1)
so the second will return all values in your db, because it checks if condition 1 or 2 is met. And condition 2 is always true.
WHERE 1
Mysql considers 1 as true
so it is simple logic- expression OR true
will always be true (so you get all rows) and expression AND true
is equivalent to expression
, so you get only the rowa that meets your condition
It looks a bit wierd, but mysql checks the expreesion (including the 1) fpr every row, even though 1 is not part of the table. It is still considered wheb decideling whether to select each row
In a room there are 10 people.
1 is a woman, the rest are men.
How many in the room are People OR Women = 10.
How many in the room are People AND Women = 1.
How many in the room are People AND Men = 9.
How many in the room are Men OR Women = 10.
How many in the room are Men AND Women = 0.
In todays day and age, this is questionable ;)
Still I hope it helps
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