A database concept used to represent missing, unknown or inapplicable data.

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3
votes
1answer
3k views

MySQL - LOAD DATA INFILE - Handling unescaped NULL in INFILE or creating INFILE with escaped NULL

I am selecting data from a database on server A, dumping the output into a file locally, and then doing a LOAD DATA INFILE on my local database. The bash command looks something like this: mysql -e ...
1
vote
2answers
1k views

null check and string concatenation

Can I do better than this query? select func_returns_str('key1') || nvl2( func_returns_str('key2'), ',' || func_returns_str('key2'), null) from dual; I ...
3
votes
2answers
2k views

PostgreSQL: count datasets / lines containing NULL values

I'm looking for a easy function to get the number of datasets (of any table) containing NULL values. It's not important how many columns are involved. For example: CREATE TABLE tab1 ( id INTEGER ...
7
votes
4answers
4k views

Finding empty columns of a table in PostgreSQL

What query would return the name of the columns of a table where all rows are NULL?
45
votes
4answers
16k views

What is the difference between select count(*) and select count(any_non_null_column)?

I seem to remember that (on Oracle) there is a difference between uttering select count(*) from any_table and select count(any_non_null_column) from any_table. What are the differences between these ...
0
votes
2answers
753 views

Is where x = 0 or x is null to be replaced by where isnull(x, 0) = 0 resp. where NVL(x, 0) = 0

Think of x as a column containing null, 0, 1. Only less than 1 % of the rows contain 1. There is no logical difference between 0 and Null. Is it a good idea to insist, that isnull rather than NVL is ...
32
votes
2answers
9k views

NULL or NOT NULL by default?

In MySQL, is it better to always allow nulls unless you know a field is required, or always use Not Null unless you know a field will contain nulls? Or doesn't it matter? I know in some DBMSs they ...
56
votes
8answers
20k views

When to use NULL and when to use an empty string?

I'm interested mainly in MySQL and PostgreSQL, but you could answer the following in general: Is there a logical scenario in which it would be useful to distinguish an empty string from NULL? What ...