I am confused and did not find in Google. Can anyone tell me What is Sql <> operator name?
<> is NOT Equal to, it's the same as !=
It's "not equal". Look in the list of operators for the database you're using, and find the appropriate section (usually "comparison operators"). For example:
SQL server
MySQL
Oracle
Postgres
It is the not equals operator.
Usage:
select *
from table
where foo <> 0
It is the Not Equal operator, but I am going to have to be verbose to get my answer posted because I haven't entered enough characters yet.
<> means not equal same as !=
Checks if the values of two operands are equal or not, if values are not equal then condition becomes true. (a <> b) or (a != b) is true.
Here is the answer – Technically there is no difference between != and <>. Both of them work the same way and there is absolutely no difference in terms of performance or result.
If != and <> both are the same, which one should be used in SQL queries?
You can use either != or <> both in your queries as both technically same but I prefer to use <> as that is SQL-92 standard.
Related
In SQL where clause,
Is
.....
AND NOT (A OR B OR C)
.....
the same as
.....
AND ((NOT A) OR (NOT B) OR (NOT C))?
.....
Thank you!
No, it's not the same. First and second will return different result. First one will return the result of the or condition.
Query below will be like the first you wrote, it will filter and return result you want.
SELECT * FROM tablename
WHERE NOT(columnname= 'columnvalue' OR columnname= 'columnvalue')
For second one, you need to use AND instead of OR. This is because you apply 'NOT' to each one.
SELECT * FROM tablename
WHERE (NOT(columnname= 'columnvalue') AND NOT(columnname= 'columnvalue'))
You can refer to this example
SQL uses ternary logic with truth tables including true, false, and unknown to handle comparisons with null. In the OR truth table, (false or unknown) == unknown (which is falsy) but (true or unknown) == true
Consider:
SELECT 1
WHERE not (1=2 or 1=null)
Versus
SELECT 2
WHERE (not 1=2) or (1=null)
Example SqlFiddle
Redgate article with more about ternary logic in SQL
Looking over code examples in my lecture slides, I notice this:
c = head();
s = null;
while (c <> null && s == null) {
if (c.value().matches("33812"))
s = c
c = next
}
Whilst I understand basically what the code does, I don't understand this particular part of the while loop condition "c <> null".
Thanks in advance!
It's almost certainly just a typo/braino for != ("not equal to"). <> is used in Basic and SQL, but not in any C-like language I'm familiar with.
c <> null is the same as c != null, which means that c is not equal to null. I can't think of any languages offhand that use it, but I know they exist. (Haskell uses <>, but it's for something completely different, and uses /= for null because it looks like the symbol ≠.)
As stated, <> means != or not equal to. The reasoning behind this is one operand cannot be equal if it is strictly greater/less than the other operand, thus <> really means all numeric possibilities EXCEPT ==, that is, !=.
<> and != mean "not equal to". SQL uses <> in general; depending on the language, this is either equivalent to != or not supported by its lexer.
<>
What does this character mean when placed in an SQL query?
That's the SQL way of writing the not equal operator.
This means "not equal" operator in SQL query ieselect * from firstname <> 'james' this means give me all rows whose first name not equal to james
It's the SQL operator for "not equal", though many databases also provide the (non standard) != operator that means exactly the same.
this means 'Not Equal Operator.' ......!!!!!
Just as != in c and other languages. <> is for SQL
this operator means not equal ....
Not equal, i.e. Less than or greater than.
I just had a trivial but genuine use for NULLIF(), for the first time in my career in SQL. Is it a widely used tool I've just ignored, or a nearly-forgotten quirk of SQL? It's present in all major database implementations.
If anyone needs a refresher, NULLIF(A, B) returns the first value, unless it's equal to the second in which case it returns NULL. It is equivalent to this CASE statement:
CASE WHEN A <> B OR B IS NULL THEN A END
or, in C-style syntax:
A == B || A == null ? null : A
So far the only non-trivial example I've found is to exclude a specific value from an aggregate function:
SELECT COUNT(NULLIF(Comment, 'Downvoted'))
This has the limitation of only allowing one to skip a single value; a CASE, while more verbose, would let you use an expression.
For the record, the use I found was to suppress the value of a "most recent change" column if it was equal to the first change:
SELECT Record, FirstChange, NULLIF(LatestChange, FirstChange) AS LatestChange
This was useful only in that it reduced visual clutter for human consumers.
I rather think that
NULLIF(A, B)
is syntactic sugar for
CASE WHEN A = B THEN NULL ELSE A END
But you are correct: it is mere syntactic sugar to aid the human reader.
I often use it where I need to avoid the Division by Zero exception:
SELECT
COALESCE(Expression1 / NULLIF(Expression2, 0), 0) AS Result
FROM …
Three years later, I found a material use for NULLIF: using NULLIF(Field, '') translates empty strings into NULL, for equivalence with Oracle's peculiar idea about what "NULL" represents.
NULLIF is handy when you're working with legacy data that contains a mixture of null values and empty strings.
Example:
SELECT(COALESCE(NULLIF(firstColumn, ''), secondColumn) FROM table WHERE this = that
SUM and COUNT have the behavior of turning nulls into zeros. I could see NULLIF being handy when you want to undo that behavior. If fact this came up in a recent answer I provided. If I had remembered NULLIF I probably would have written the following
SELECT student,
NULLIF(coursecount,0) as courseCount
FROM (SELECT cs.student,
COUNT(os.course) coursecount
FROM #CURRENTSCHOOL cs
LEFT JOIN #OTHERSCHOOLS os
ON cs.student = os.student
AND cs.school <> os.school
GROUP BY cs.student) t
I have the following SQL query:
select AuditStatusId
from dbo.ABC_AuditStatus
where coalesce(AuditFrequency, 0) <> 0
I'm struggling a bit to understand it. It looks pretty simple, and I know what the coalesce operator does (more or less), but dont' seem to get the MEANING.
Without knowing anymore information except the query above, what do you think it means?
select AuditStatusId
from dbo.ABC_AuditStatus
where AuditFrequency <> 0 and AuditFrequency is not null
Note that the use of Coalesce means that it will not be possible to use an index properly to satisfy this query.
COALESCE is the ANSI standard function to deal with NULL values, by returning the first non-NULL value based on the comma delimited list. This:
WHERE COALESCE(AuditFrequency, 0) != 0
..means that if the AuditFrequency column is NULL, convert the value to be zero instead. Otherwise, the AuditFrequency value is returned.
Since the comparison is to not return rows where the AuditFrequency column value is zero, rows where AuditFrequency is NULL will also be ignored by the query.
It looks like it's designed to detect a null AuditFrequency as zero and thus hide those rows.
From what I can see, it checks for fields that aren't 0 or null.
I think it is more accurately described by this:
select AuditStatusId
from dbo.ABC_AuditStatus
where (AuditFrequency IS NOT NULL AND AuditFrequency != 0) OR 0 != 0
I'll admit the last part will never do anything and maybe i'm just being pedantic but to me this more accurately describes your query.
The idea is that it is desireable to express a single search condition using a single expression but it's merely style, a question of taste:
One expression:
WHERE age = COALESCE(#parameter_value, age);
Two expressions:
WHERE (
age = #parameter_value
OR
#parameter_value IS NULL
);
Here's another example:
One expression:
WHERE age BETWEEN 18 AND 65;
Two expressions
WHERE (
age >= 18
AND
age <= 65
);
Personally, I have a strong personal perference for single expressions and find them easier to read... if I am familiar with the pattern used ;) Whether they perform differently is another matter...