Query: SELECT id, name, FROM users u WHERE **id <> 0** LIMIT 50 OFFSET 0
What does the clause id <> 0 mean here? Does it mean:
id is less than zero or id is greater than zero
<> means "not equal" (it can also be written as != with some DBMS)
It means not equal and apparently I have to submit at least 30 characters for my answer.
It means, "where ID is different from 0".
So, both greater than or less than 0.
it means that you are fetching all records with an id different than 0 (zero), I've sometimes used this just to check if some record is already saved (if the record has an ID it means is saved).
It means only include results who's field id has a value of greater or smaller than 0, basically records that have non-zero id's - but really this should not be possible, if it is then I'd recommend reconsidering your table designs.
Related
I have the following query that is part of a common table expression. I don't understand the function of the "Select -1" statement. It is obviously different than the "Select 1" that is used in "EXISTS" statements. Any ideas?
select days_old,
count(express_cd),
count(*),
case
when round(count(express_cd)*100.0/count(*),2) < 1 then '0'
else ''
end ||
cast(decimal(round(count(express_cd)*100.0/count(*),2),5,2) as varchar(7)) ||
'%'
from foo.bar
group by days_old
union all
select -1, -- Selecting the -1 here
count(express_cd),
count(*),
case
when round(count(express_cd)*100.0/count(*),2) < 1 then '0'
else ''
end ||
cast(decimal(round(count(express_cd)*100.0/count(*),2),5,2) as varchar(7)) ||
'%'
from foo.bar
where days_old between 1 and 7
It's just selecting the number "minus one" for each row returned, just like "select 1" will select the number "one" for each row returned.
There is nothing special about the "select 1" syntax uses in EXISTS statements by the way; it's just selecting some random value because EXISTS requires a record to be returned and a record needs data; the number 1 is sufficient.
Why you would do this, I have no idea.
When you have a union statement, each part of the union must contain the same columns. From what I read when I look at this, the first statement is giving you one line for each days old value and then some stats for each day old. The second part of the union is giving you a summary of all the records that are only a week or so less. Since days old column is not relevant here, they put in a fake value as a placeholder in order to do the union. OF course this is just a guess based on reading thousands of queries through the years. To be sure, I would need to actually run teh code.
Since you say this is a CTE, to really understand why this is is happening, you may need to look at the data it generates and how that data is used in the next query that uses the CTE. That might answer your question.
What you have asked is basically about a business rule unique to your company. The true answer should lie in any requirements documents for the original creation of the code. You should go look for them and read them. We can make guesses based on our own experience but only people in your company can answer the why question here.
If you can't find the documentation, then you need to talk (Yes directly talk, preferably in person) to the Stakeholders who use the data and find out what their needs were. Only do this after running the code and analyzing the results to better understand the meaning of the data returned.
Based on your query, all the records with days_old between 1 and 7 will be output as '-1', that is what select -1 does, nothing special here and there is no difference between select -1 and select 1 in exists, both will output the records as either 1 or -1, they are doing the same thing to check whether if there has any data.
Back to your query, I noticed that you have a union all and compare each four columns you select connected by union all, I am guessing your task is to get a final result with days_old not between 1 and 7 and combine the result with day_old, which is one because you take all between 1 and 7.
It is just a grouping logic there.
Your query returns aggregated
data (counts and rounds) grouped by days_old column plus one more group for data where days_old between 1 and 7.
So, -1 is just another additional group there, it cannot be 1 because days_old=1 is an another valid group.
result will be like this:
row1: days_old=1 count(*)=2 ...
row2: days_old=3 count(*)=5 ...
row3: days_old=9 count(*)=6 ...
row4: days_old=-1 count(*)=7
I am trying to accumulate a value when a certain condition exists such as
If statusCode = 0
then add 1 to a value.
I am trying to show the number of successful records as defined by the statusCode.
There must be a better way to do this.
Thanks
Select count(1) from yourTable where statusCode=0
My table name is products and i am trying to update products_price so all prices have 99p at the end. Can you help?
Also this request is a bit simpler (i'm new to SQL!) I want to update all records as follows
anything in categories where parent_id is >0 I need the script to update all records to zero.
Q1
So, you want all prices to be truncated and then add 0.99 currency units. The function to truncate a value to 0 decimal places varies between DBMS; I'm assuming it is called TRUNC, but it might be called FLOOR or something else in your DBMS:
UPDATE Products
SET Products_Price = TRUNC(Products_Price) + 0.99;
You don't need a WHERE clause here. I'm assuming there are no negative prices; if that's a possibility, then you need to be a little cleverer with your assignment (maybe a CASE expression; maybe you have a function SIGN, SIGN, SIGNUM available).
Note that I'm assuming that the p referenced is pence, as in 'pounds (Sterling) and pence'. If the price was between £0.00 and £0.99, the result will be £0.99; if the price was between £1.00 and £1.99, the result will be £1.99; etc. The TRUNC or FLOOR will remove any fractional part from the price, and the addition of 0.99 then means that the result is of the form £x.99 for (non-negative) integer values of x.
Q2
Making some assumptions about what you meant:
UPDATE Categories
SET Anything = 0
WHERE Anything > 0
AND Parent_ID > 0;
If you meant that you want to update the products table in some way, then you need to be a lot more precise in your question.
Q2 revisited
From a comment:
I want to update all parent_id values in my categories table to zero where parent_id > 0.
Presumably, there must be some parent_id values which are null or negative, so you need:
UPDATE Categories
SET Parent_ID = 0
WHERE Parent_ID > 0
If there are no nulls and no negatives, then you can run an even simpler update:
UPDATE Categories
SET Parent_ID = 0
It will set to zero those rows which already have a parent ID of zero as well as those that do not. This might hit the logs a bit harder than the more selective update (as in, there will be more changes to record in the logical log or equivalent for your DBMS), but unless there are vast numbers of records and most of them already have a zero parent ID and the DBMS does not recognize when a record does not change and writes log records for unchanged rows, then you're unlikely to notice the difference
Set price to have 99p at end:
update products_price set price = floor(price)+0.99;
Set parent_id to 0:
update categories set parent_id = 0 where parent_id > 0;
What does <> mean in SQL language: Sample code is as follows
SELECT ordid,
prodid,
qty
FROM item
WHERE prodid IN (SELECT prodid
FROM item
WHERE ordid = 605)
AND qty IN (SELECT qty
FROM item
WHERE ordid = 605)
AND ordid <> 605;
It means 'not equal to'. So you're filtering out records where ordid is 605. Overall you're looking for any records which have the same prodid and qty values as those assigned to ordid 605, but which are for a different order.
Does not equal. The opposite of =, equivalent to !=.
Also, for everyone's info, this can return a non-zero number of rows. I see the OP has reformatted his question so it's a bit clearer, but as far as I can tell, this finds records where product ID is among those found in order #605, as is quantity, but it's not actually order #605. If order #605 contains 1 apple, 2 bananas and 3 crayons, #604 should match if it contains 2 apples (but not 3 dogs). It just won't match order #605. (And if ordid is unique, then it would find exact duplicates.)
It just means "different of", some languages uses !=, others (like SQL) <>
not equals. See here for a list of conditions
I'm surprised nobody mentioned the null special case. I think the meaning of <> is more something like
has a value which is not equal to
In this case it filters out items which have ordid 605 and items which have a null ordid.
It may be obvious in this context that ordid is never null, but it is never hurts to remember that null is not <> from 605 (or from anything).
It means not equal to
Should I use != or <> for not equal in TSQL?
Have a look at the link. It has detailed explanation of what to use for what.
It (<>) is a function that is used to compare values in database table.
!= (Not equal to) functions the same as the <> (Not equal to) comparison operator.
It means not equal to, this is a good method to exclude certain elements from your query. For example lets say you have an orders tables and then you have OrderStatusID column within that table.
You also have a status table where
0 = OnHold,
1 = Processing,
2 = WaitingPayment,
3 = Shipped,
4 = Canceled.
You can run a query where
Select * From [Orders] where OrderStatusID <> 4
this should give you all the orders except those that have been canceled! :D
I have a query:
select *
from randomtable
where randomnumber <> 0
The column "random_number" will never be a negative number.
So, if I write the query as:
select *
from randomtable
where randomnumber > 0
Is there any major difference?
No, there is no difference at all (in your specific situation). All numeric comparisons take the same time.
What's done at the lowest level is that zero is subtracted from randomnumber, and then the result is examined. The > operator looks for a positive non-zero result while the <> operator looks for a non-zero result. Those comparisons are trivial, and take the same amount of time to perform.
If it will never be less than zero, then NO
The important thing is to determine how you know that random_number will never be a negative number. Is there a constraint that guarantees it? If not, what do you want your code to do if a bug somewhere else causes it to be negative?
The result set should never be different. The query path might be as the second might choose an index range scan starting at randomnumber=0 and looking for the records in sequence. As such the order of the results may differ.
If the order of the results is important, then put in an ORDER BY
What does 'greater than' mean? It means 'not equal' AND 'not less than'. In this case, if the number cannot be less than zero, 'greater than' is equivalent to 'not equal'.