SQL:How to compare with NULL without "IS NULL" operator

#sql #null #rdbms #database

We use IS NULL instead of =for comparison with NULL. This is because there are many RDBMSs that become UNKNOWN if you compare null and value with =.

comparison of some value and null to false
comparison of null and null to true

Some times we want to compare it like that. In such a case we can use the comparison operator IS DISTINCT FROM orIS NOT DISTINCT FROM.

A	B	A = B	A IS NOT DISTINCT FROM B
0	0	true	true
0	1	false	false
0	null	unknown	false
null	null	unknown	true

environment and version

postgres=# SELECT version();
                                                             version                                                              
----------------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 10.1 on x86_64-apple-darwin13.4.0, compiled by Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn), 64-bit
(1 row)

$ sqlite3 --version
3.22.0 2018-01-22 18:45:57 0c55d179733b46d8d0ba4d88e01a25e10677046ee3da1d5b1581e86726f2171d

Example on PostgreSQL

--compare null and value with '='( result is unknown)
postgres=# select null = 4;
 ?column? 
----------

(1 row)

--compare value and value (result is true/false)
postgres=# select 4 = 4;
 ?column? 
----------
 t
(1 row)

--compare null and value with 'IS DISTINCT FROM' operator (result is true/false)
postgres=# select null is distinct from  4;
 ?column? 
----------
 t
(1 row)

-- use 'IS NOT DISTINCT FROM' to check equal
postgres=# select null is not distinct from  4;
 ?column? 
----------
 f
(1 row)

-- You can also compare values and values using 'IS DISTINCT FROM'
postgres=# select 4 is distinct from  4;
 ?column? 
----------
 f
(1 row)

sqlite: Use `IS` instead of `IS DISTINCT FROM`

sqlite can't use IS DISTINCT FROM.

You can compare with IS instead

sqlite> select 4 = 4;
1
sqlite> select 4 is null;
0
sqlite> select 4 is 4;
1
sqlite> select 4 is 5;
0
sqlite> select null is null;
1

other

IS DISTINCT FROM operator in MySQL is <=>

reference

https://modern-sql.com/feature/is-distinct-from
Joe Celko's SQL for Smarties: Advanced SQL Programming(Japanese version) 16. 3 IS [NOT] DISTINCT FROM operator