HAVING clause properties

https://stackoverflow.com/questions/5317600

24-10-2019
|

Question

I need help with this exercise where I have to translate properties from English into sql.

Questions:

length has at least one 0
```
Answer: MIN(ABS(length)) = 0
```
length has more that one value
```
Answer: MIN(length) < MAX(length)
```
there are no nulls for length
```
Answer: COUNT(*) = COUNT(length)
```
values of length are either positive or negative
```
Answer: MIN(length) * MAX(length) > 0
```
length differs above and below zero by the same amount
```
Answer: MIN(length) = -max(length)
```

all values of length are different

Answer: COUNT(DISTINCT length) = COUNT(length)

only one of min(length) or max(length) is positive
```
Answer: MIN(length) * MAX(length) < 0 
```
length has one value or null value
```
Answer: MIN(length) = MAX(length)
```

Are these correct? I'm not sure about #3, 5, 6 and 8 in particular.

Solution

length has at least one 0
```
Answer: MIN(ABS(length)) = 0
```

length has more that one value

Answer: MIN(length) < MAX(length)
or
Answer: COUNT(distinct length) > 1

there are no nulls for length
```
Answer: COUNT(*) = COUNT(length)
```

values of length are either positive or negative

Answer: MIN(length) * MAX(length) > 0
or
Better Answer: Sign(MIN(length)) * Sign(MAX(length)) > 0

-- Because, with large values of length, multiplying two of them
-- together can give you an overflow.

length differs above and below zero by the same amount
```
Answer: MIN(length) = -max(length)
```

all values of length are different

Answer: COUNT(DISTINCT length) = COUNT(length)

only one of min(length) or max(length) is positive

Maybe Wrong Answer: MIN(length) * MAX(length) < 0 (0 is not positive)
or
Answer: MIN(length) <= 0 and MAX(length) > 0

-- IF: one must be positive and the other must be negative
-- change <= 0 to < 0

length has one value or null value

Wrong Answer: MIN(length) = MAX(length) -- null=null does not return true
or
Answer: COUNT(distinct length) <= 1

OTHER TIPS

I would question #2. You may have two identical length values.

For #3, I would not use the HAVING clause to check for non-nulls. Same thing for #4. HAVING is not the right place to be checking equivalency for each row against a scalar value.

#5 might have some problems worth wording. Are you certain they mean min and max and not distribution?

#8 is oddly worded as well. Null could be the value of length, or it could be absence of value. It depends on how the query is formated. Also, is there a requirement that you not use operators like AND & OR?

Also, NULL is treated as "unknown" with most database platforms. Since it is unknown, it cannot be checked for equivalency against another value. If you don't know what Value1 is, you can't possibly know if it's the same as what's in Value2...

Licensed under: CC-BY-SA with attribution

Not affiliated with StackOverflow