How does OR operator works in SQL? - mysql

Question

Here is the query which make me to thought that how does OR operator in SQL works.
SELECT * FROM crop WHERE `crop_id`=1 OR ''='' ;
what i assumed that out put will be with records having crop_id equals to 1 because I have records with that value, and logically it does not evaluates the 2nd condition.
But it is returning all the records, rather than returning filtered(with crop_id) values.
Am I taking it wrong?
Update
I have a condition that if crop_id=1 matches it should return all the values having crop_id = 1 other wise return all records.
Upadte 2
Query mentioned above have both conditions are true, i.e crop_id=1 and ''='' I don't want alternatives, I just want to know that why it is ignoring first condition and taking 2nd condition.

Answer 1

OR
it works as in any boolean statement.
what i assumed that out put will be with records having crop_id equals to 1 because I have records with that value, and logically it does not evaluates the 2nd condition.
You are right, but if you don't have value equal to 1 then it evaluates the second part and it returns true and hence you always get result, either you match or not to your first part of OR.

Answer 2

The boolean logic of OR operator is as below:
true OR true = true
false OR true = true
true OR false = true
false OR false = false
So in your SQL query, you put '' = '' so your boolean logic evaluates to true for this, so you will get all results for this too. If you want to get results with crop_id = 1 only, then use below query:
SELECT * FROM crop WHERE `crop_id`=1
There is no need to use the OR.

Answer 3

For your original question the answer is:
WHERE something OR TRUE -- always TRUE
So you will get always all records.
For second question it looks like you want:
You pass in #param value specific value e.g. 1 or NULL;
SELECT *
FROM crop
WHERE crop_id = #param OR #param IS NULL;
or
SELECT *
FROM crop
WHERE crop_id = COALESCE(#param, crop_id);
You can also use IF ELSE for this like:
IF #param IS NULL THEN
SELECT *
FROM crop
ELSE
SELECT *
FROM crop
WHERE crop_id = #param
END IF
EDIT:
People says you don't need to know Math to be programmer.
This is exactly the case when Math(Boolean Logic) is needed.
╔═════════════╦═════════════╦═══════╗
║ Statement_1 ║ Statement_2 ║ AND ║
╠═════════════╬═════════════╬═══════╣
║ TRUE ║ TRUE ║ TRUE ║
║ FALSE ║ TRUE ║ FALSE ║
║ TRUE ║ FALSE ║ FALSE ║
║ FALSE ║ FALSE ║ FALSE ║
╚═════════════╩═════════════╩═══════╝
╔═════════════╦═════════════╦═══════╗
║ Statement_1 ║ Statement_2 ║ OR ║
╠═════════════╬═════════════╬═══════╣
║ TRUE ║ TRUE ║ TRUE ║
║ FALSE ║ TRUE ║ TRUE ║
║ TRUE ║ FALSE ║ TRUE ║
║ FALSE ║ FALSE ║ FALSE ║
╚═════════════╩═════════════╩═══════╝
Now Statement_2 is '' = '' which is always TRUE
╔═════════════╦═════════════╦══════╗
║ Statement_1 ║ Statement_2 ║ OR ║
╠═════════════╬═════════════╬══════╣
║ TRUE ║ TRUE ║ TRUE ║
║ FALSE ║ TRUE ║ TRUE ║
╚═════════════╩═════════════╩══════╝
As you see the result is always TRUE. And there is no need to evaluate Statement_1.
condition OR TRUE <=> TRUE (THIS IS TAUTOLOGY!!!)
Tautology is a formula that is true in every possible interpretation
And finally your condition is equivalent to:
WHERE 1=1 -- will return all records

Answer 4

for your desired output or will not be used rather if else will be used. i.e.
you have to
select count (*) from crop where 'crop_id'=1
if count>0
{
select * from crop where 'crop_id'=1
}else
{
select * from crop;
}
because or works like it will give all the results of equal to 1 and all the empty ones as well.
as or means either of the condition should be true.
so either it is 1 then it will come.
if it is empty than it will come
hence all will come.
so you have to use if else.
please do a google for the proper if else syntax. but logic will be these only.
thanks

Answer 5

The OR operator checks if any of the expressions are true, then it would return true.
In your case
''=''
is always true so the SELECT query returns all the records.