There are algorithms to convert truth table with multiple variable to a DNF (Disjunctive normal form). Is there a similar concept which can be generalized for variables taking a limited set of values?
For example, from this truth table we can compose DNF:
(¬A∧¬B∧¬D) ∨ (¬A∧B∧C) ∨ (A∧B∧D) ∨ (A∧¬B∧¬C)
Is there any automatic tools or algorithms which might help me to convert tables like this:
into the list of predicates composed of OR, AND, = ?
Result:
(a = A2) -> 1
((a = A3) AND (b = B2)) -> 1
otherwise -> 0
Each variable have a limited set of possible values. In my case I have 3 variable, each of them can take up to 30 different values.
Related
How can i get only Lines, where 2 (or more) Where Clause of many are true?
For Example:
SELECT *
FROM A
WHERE CONVERT(AES_DECRYPT(name,'".$this->key."'),CHAR) LIKE '%".$this->escape($name)."%'"
OR CONVERT(AES_DECRYPT(name2,'".$this->key."'),CHAR) = '".$this->escape($name2)."'"
OR CONVERT(AES_DECRYPT(name3,'".$this->key."'),CHAR) = '".$this->escape($name3)."'"
OR CONVERT(AES_DECRYPT(name4,'".$this->key."'),CHAR) = '".$this->escape($name4)."'"
.....
is there a simply way to get only the results where 2 (or more) match, without creating a huge SQL Statement (each column with each other column - (name AND name2) OR (name AND name3) OR (name AND name4) OR ... )
In MySQL you can utilise the fact that it treats boolean expressions as 1 or 0 in a numeric context. Thus to check for 2 or more conditions being true, you can write
WHERE (condition 1) + (condition 2) + ... + (condition n) >= 2
Note that the parentheses around each condition are required to prevent any operator precedence issues.
I use Chartio to create dashboards. I'm able to use variables with Chartio that can fill in sections of a MySQL query and then pump out a cool looking graph. I have a situation where I need a query that can have any combination of 3 variables X, Y, Z as shown below.
SELECT orderid
FROM orders
WHERE productcode IN (X) AND
status IN (Y) AND
date IN (Z);
I need to have the ability for the query to "determine" that if I only give it X, ignore Y and Z as a condition, for example. Or if I give it X and Y, ignore Z. I could give it any combinations of those three. By "ignore" I mean not use it as a condition in the WHERE clause.
Is this possible using OR? REGEXP? Wildcards? ...? I'm not very well versed in MySQL. Thanks in advance
if it sets the variable to an empty string when the user leaves the field out, you can write:
SELECT orderid
FROM orders
WHERE (X = '' OR productcode = X) AND
(Y = '' OR status = Y) AND
(Z = '' OR date = Z);
I would like to substract 1 from SEVERAL elements in a table if they are not equal to 0.
Code for one element
UPDATE `database` SET a1=a1-1 WHERE a1!='0' AND username='bob';
I want to do the same thing on the same command for several elements, a2, a3... (username stay the same for all elements)
Thank you
You're probably looking for conditionals within your set expressions. This can be achieved with the keyword CASE:
UPDATE 'database'
SET
a1 = CASE WHEN a1 != 0 THEN (a1 - 1) END,
a2 = CASE WHEN a2 != 0 THEN (a2 - 1) END
WHERE
username='bob';
Be sure to understand the principles of the sql update syntax which consists basically of a single UPDATE ... SET ... [WHERE] pattern.
Hint: I'm pretty sure your a1,a2,a3,.. fields are of type integer, so - unlike in your code example - the value to check must not put into quotes!
Given 3 possible values for X (A, B, C), is it faster to do:
WHERE (X = 'B' OR X = 'C'), or
WHERE X != 'A'
Or does it depend? If so, then what does it depend on?
Option 1:
WHERE (X = 'B' OR X = 'C')
and
WHERE X IN ('B', 'C')
are equivalent and may use an index on (X).
Option 2:
WHERE X != 'A'
will not use an index on (X). See a comment by Henrik Grubbström at the MySQL docs, How MySQL Optimizes WHERE Clauses page:
Indexes are ignored for the <> operator:
So, if the use of index makes the query faster (for example, if 99% of the table has X = 'A'), use the first option.
Note: The != operator is a synonym (in MySQL) of the SQL-standard <> inequality operator.
Your second operation should be faster because it requires one less logical check. If it's scanning a value, it only has to check to make sure it's not A, where your first operation would need to match B and then if there is no match, C. Regarding the use of an index, it depends on what your index looks like and how it's being called. If you have an index on columns W, X and you only filter X, the index will not be used as indexes work left-to-right.
Direct equality (=) and inquality (!=) takes the same time. Best case your queries will run the same time, but worst case, case 1. could be slower as you're adding another case to check by the OR.
Of course not knowing if there are indexes or the distribution of values for X can affect the performance...
In my opinion the second item is better because it is always only one comparison; in the first item if the value to be tested is 'C' or 'A' you have to 2 comparison, the fisrt (X = 'B') will fail and then the second comparison gives the final result.
If case 1 uses an index, which in my view it should if there is an index on X, it will be faster than case 2 if case 2 doesn't use an index, which in my view it won't. In general. It also depends on the actual distribution of values: if significantly skewed, results will vary accordingly.
I have no idea if this is the right forum or not. Lets say I have the following:
SELECT *
FROM MyTable m
WHERE ((A OR B) AND (C OR D))
Assume that A, B, C, D are proper boolean clauses that each need to be evaluated on a row-level basis. Lets also assume no indexes.
This is logically equivalent to:
SELECT *
FROM MyTable m
WHERE (A AND C)
OR (A AND D)
OR (B AND C)
OR (B AND D)
Is there a performance advantage to either one? We're on MSSql-2008.
My understanding is that your first case is more efficient, because:
in this clause:
WHERE ((A OR B) AND (C OR D))
the entire statement fails if neither A or B are true; the Second part of the statement, (C OR D) is not evaluated. Even if A OR B are true, there is only one more pair to check - C OR D. Worst case is that four criteria are checked before the statement as a whole can be evaluated (if A = False, B = False, C= False, but D = True). Best case is, the statement becomes false after checking only A and B. If neither are true, then the entire statement is false.
In your second case, each of the four cases must ALL be evaluated before the statement as a whole can be evaluated.
Nesting the OR conditionals inside the AND means if the first case fails, more on along, nothing more of interest here. You improve things even more if you place the case most likely to be false as the first pair.
I will be interested to hear from others on this . . .