I have created a table called table1 and it has 4 columns named Name,ID,Description and Date.
I have created them like Name varchar(50) null, ID int null,Description varchar(50) null, Date datetime null
I have inserted a record into the table1 having ID and Description values. So Now my table1 looks like this:
Name ID Description Date
Null 1 First Null
One of them asked me to modify the table such a way that The columns Name and Date should have Null values instead of Text Null. I don't know what is the difference between those
I mean can anyone explain me the difference between these select statements:
SELECT * FROM TABLE1
WHERE NAME IS NULL
SELECT * FROM TABLE1
WHERE NAME = 'NULL'
SELECT * FROM TABLE1
WHERE NAME = ' '
Can anyone explain me?
In a CREATE TABLE, the NULL or NOT NULL here varchar(50) null is a constraint that determines whether NULLs are allowed. NOT NULL means no.
When you inserted data, which statement did you run?
INSERT TABLE1 VALUES (Null, 1, First, Null)
or
INSERT TABLE1 VALUES ('Null', 1, First, 'Null')
The first one uses the keyword NULL, inserts a NULL (not a null value: no such thing, arguably). No values is stored except in the NULL bitmap fields
The second one has a string "null" and the characters N, U, L, L + 2 bytes for length are stored
When you run SELECT * FROM TABLE1, client tools will show NULL.
To test whether you actually have NULLs or the string NULL, run this
SELECT ISNULL(name, 'fish'), ISNULL(date, GETDATE()) FROM TABLE1
For the SELECTs
--null symbols. No value stored
SELECT * FROM TABLE1 WHERE NAME IS NULL
--string null
SELECT * FROM TABLE1 WHERE NAME = 'NULL'
--empty string
SELECT * FROM TABLE1 WHERE NAME = ' '
Note: null symbol/value is not empty string. It has no value and won't compare. Even to itself.
As for your DBA, the code above with ISNULL will decide what is stored.
Edit: if you are storing null symbol/value, then your DBA should read up on "null bitmap"
The data does represent nulls. The text 'Null' is your query tool displaying the text.
One of them asked me to modify the table such a way that The columns Name and Date should have Null values instead of Text Null. I don't know what is the difference between those.
The NULL keyword indicates the absence of any value -- the value is unknown.
But that won't stop someone from storing the letters that spell out "NULL", data type providing (which INT and DATETIME will not). Because of this, operators like IS NULL would not work on text that spells out "NULL" and vice versa -- searching for strings using: ... LIKE '%NULL%' will not return records with NULL values.
The data type of the column does matter with regard to NULL in SQL Server. In a UNION statement, you need to cast NULL to be the appropriate data type -- the default for NULL is INT:
SELECT CAST('2011-01-01 00:00:00' AS DATETIME)
UNION
SELECT CAST(NULL AS DATETIME)
Based on the information provided about the columns and the output, the DBA appears to be asking you to change the text the client you are using to connect to SQL Server with displays when a NULL value is encountered in a resultset. Reminds me of my first job dishwashing, and was asked to get the lefthanded spatula...
The string "Null" is a string.
The value of NULL (or Null or null, SQL is case-insensitive when it comes to these things) is used to denote an unknown value. It's the empty set of values, if you will.
http://www.w3schools.com/sql/sql_null_values.asp
NULL, in software, is symbolic of no value. Assuming you're inserting the columns using a string with null as the value, use the null constant. e.g.
INSERT INTO table1 (Name,ID,Description,Date) VALUES (NULL,1,'First',NULL);
Note that NULL is a constant in SQL, not the word "NULL" in a string.
AFAIC, there is no different between NULLs. There are different column types. But as long as a column is a text data type, and it's NULL, it's a text NULL.
Sometimes there are questions about empty strings ("") instead of NULLs, but the description you're using doesn't seem to be referring to that.
SELECT * FROM TABLE1 WHERE NAME IS NULL
Returns all rows where the Name is NULL
SELECT * FROM TABLE1 WHERE NAME = 'NULL'
Returns all rows where the Name is equal to the string 'NULL', Null values are not returned
SELECT * FROM TABLE1 WHERE NAME = ' '
Returns all rows where the Name is equal to exactly one space ' ', Null values are not returned
If you run this statement it might help clear up when its null and when its not
select
*,
case
WHEN name is null THEN 'Its Null alright'
ELSE 'It has a value'
END
FROM TABLE1
Related
Look my example, what's the difference between two codes?
Select name from customers where name is not null
Select name from customers where name <> ''
They do completely different things.
Select name from customers where name is not null
This one selects any customer who has a value in the name field. Those values can include '' as well as things like 'Sam', 'John Jones', 'pretty blonde girl'.
Select name from customers where name <> ''
This will select all names that are not null or blank In Sql Server at least. Other databases may handle this differently. The reason why it also excludes Null is that Null cannot be part of a comparison since it by definition means we don't have a clue what the value of this field is.
If you wanted to return both real names and null values and only exclude the empty strings. In SQl Server you would do:
Select name from customers where coalesce(name, 'Unknown') <>''
There are a lot of correct answers here but I think you are missing what NULL is. It's nothing so it's not comparable to anything. Here's some test for you
DECLARE #param CHAR(1)=NULL --you can replace #param with your column name in your queries
SELECT 1 WHERE #param = NULL --you can't compare NULL to anything using = > < <> != or any other comparision operator
SELECT 1 WHERE #param = '' --an empty value isn't the same as a NULL value so if a NULL is present it won't be returned
SELECT 1 where #param IS NULL --this is how you have to check for null values
--If you want to check for both empty and nulls, you can force the empty string with COALESCE or ISNULL
SELECT 1 WHERE COALESCE(#param,'') = ''
SELECT 1 WHERE ISNULL(#param,'') = ''
The key concept you are missing is that in SQL Server, NULL does not mean no value, it means that the value is unknown. So consider your query with some silly sample data:
DECLARE #t TABLE
( id INT IDENTITY(1,1) PRIMARY KEY CLUSTERED
, name VARCHAR(10) NULL );
INSERT INTO #t
VALUES
('dog'),
('cat'),
(''),
(NULL);
SELECT *
FROM #t
WHERE name <> '';
What you are asking the engine is to return the records where name is not an empty string. On the evaluation of the fourth record, it is determining if NULL equals empty string or not. NULL could be empty string, we don't know ... given that the value is unknown, the engine can't include that record because you are asking for only records where we know the name is definitely not empty string.
Consider another query against the same data:
WITH cteTemp AS
(
SELECT *
, isEqualToEmptyString = CASE WHEN name = '' THEN 'true' ELSE 'false' END
FROM #t
)
SELECT *
FROM cteTemp
WHERE isEqualToEmptyString = 'false';
Now this is written to demonstrate a point and there are cleaner ways to do the same thing (such as the COALESCE in HLGEM's answer.) But understand what is happening here: the query is first determining for sure what names are empty string (which excludes the NULL since its value is unknown) and then excluding those that are. Thus, the NULL is returned.
Null and empty string are two different things. A given field in a table can either have no value (null) or a value of an empty string (''). The results return by each query will be mutually exclusive.
Is not null determines if the object/record being inspected is a true null value or not (no data).
<> '' means is not an empty string, so the record does contain data (that there is an empty string) and is not actually null.
So a query with is not null will return records with a value of string.empty, and a query with <> '' will return values that are neither null nor empty. To catch both empty strings and null values you should use <> '' in your SQL statements.
I have a Table BoxTrans
the table Contain Rows (ID,Date,FromBox,ToBox,Value)
I want to make a View like (ID,Date,Box,ValueIn,ValueOut)
select when frombox Give Value to ValueOut
and when tobox Give Value to ValueIN
You can use a CASE statement to check the value of a different column when populating a column. The below query will return your output as long as either ToBox or FromBox is NULL, if they are both not null you may get unexpected results.
SELECT ID,
Date,
COALESCE(ToBox,FromBox) as Box,
CASE WHEN ToBox IS NOT NULL THEN value ELSE NULL as ValueIn,
CASE WHEN FromBox IS NOT NULL THEN value ELSE NULL as ValueOut
FROM BoxTrans
Let's say I want to display all items in a table with following criteria how would i do this?
SELECT *
FROM TABLE
WHERE TABLE.COLUMN1 = 'example' AND TABLE.COLUMN2 != 'NULL'
I want it to display all values from COLUMN1. How does one go about this process in MS SQL?
NULL values can be compared using IS [NOT] NULL in SQL server. Please check this.
SELECT *
FROM TABLE
WHERE TABLE.COLUMN1 = 'example' AND TABLE.COLUMN2 IS NOT NULL
NULL is an UNKNOWN value , you cannot use any Comparison Operators (= , <> , > , <) with it. you check for nulls like
ColumnName IS NULL or ColumnName IS NOT NULL
If you think about it , it makes sense, to compare two or more values, you need to know the values only then you can compare them, Since SQL Server considers a NULL to be an UNKNOWN value, you cannot really compare an unknown value to anything.
How do you write a SELECT statement that only returns rows where the value for a certain column is null?
Do you mean something like:
SELECT COLUMN1, COLUMN2 FROM MY_TABLE WHERE COLUMN1 = 'Value' OR COLUMN1 IS NULL
?
I'm not sure if this answers your question, but using the IS NULL construct, you can test whether any given scalar expression is NULL:
SELECT * FROM customers WHERE first_name IS NULL
On MS SQL Server, the ISNULL() function returns the first argument if it's not NULL, otherwise it returns the second. You can effectively use this to make sure a query always yields a value instead of NULL, e.g.:
SELECT ISNULL(column1, 'No value found') FROM mytable WHERE column2 = 23
Other DBMSes have similar functionality available.
If you want to know whether a column can be null (i.e., is defined to be nullable), without querying for actual data, you should look into information_schema.
Use Is Null
select * from tblName where clmnName is null
You want to know if the column is null
select * from foo where bar is null
If you want to check for some value not equal to something and the column also contains null values you will not get the columns with null in it
does not work:
select * from foo where bar <> 'value'
does work:
select * from foo where bar <> 'value' or bar is null
in Oracle (don't know on other DBMS) some people use this
select * from foo where NVL(bar,'n/a') <> 'value'
if I read the answer from tdammers correctly then in MS SQL Server this is like that
select * from foo where ISNULL(bar,'n/a') <> 'value'
in my opinion it is a bit of a hack and the moment 'value' becomes a variable the statement tends to become buggy if the variable contains 'n/a'.
select Column from Table where Column is null;
select * from tableName where columnName is null
For some reasons IS NULL may not work with some column data type. I was in need to get all the employees that their English full name is missing, I've used:
SELECT emp_id, Full_Name_Ar, Full_Name_En
FROM employees
WHERE Full_Name_En = '' or Full_Name_En is null
Basically I want to alter the boolean value selecting from the table:
e.g.:
SELECT otherColumns, not ysnPending FROM table
I need a column ysnPending = true if the value is false & false if the value is true.
Is there any function available to alter the Boolean value or I should use IIf or CASE...?
use CASE, or if the bit field is non-nullable you could just subtract from 1.
SELECT
otherColumns,
(1 - ysnPending) -- NOT ysnPending
FROM table
(Using CASE might lead to more understandable code.)
If ysnPending is nullable, what behaviour do you assign to NOT?
Example using a case statement :
create table table1 (id int not null, ysnPending bit null)
insert table1 values (1, 1)
insert table1 values (2, null)
insert table1 values (3, 0)
select id, cast((case when ysnPending = 1 then 0 else 1 end) as bit) as Not_ysnPending from table1
Assumes you want 1 returned when ysnPending is NULL.
The cast to bit type is to make sure that the returned column is of a BIT datatype. If you leave it out, it will return an INTEGER type. (This may or may not matter to you, depending on how exactly you are going to use the returned result set).