sql - is null vs. equals null -


i have rough understanding of why = null in sql , is null not same, questions this one.

but then, why is

update table  set column = null

a valid sql statement (at least in oracle)?

from answer, know null can seen "unknown" , therefore , sql-statement where column = null "should" return rows, because value of column no longer an unknown value. set null explicitly ;)

where wrong/ not understand?

so, if question maybe unclear: why = null valid in set clause, not in where clause of sql statement?

sql doesn't have different graphical signs assignment , equality operators languages such or have. in such languages, = assignment operator, while == equality operator. in sql, = used both cases, , interpreted contextually.

in where clause, = acts equality operator (similar == in c). i.e., checks if both operands equal, , returns true if are. mentioned, null not value - it's lack of value. therefore, cannot equal other value.

in set clause, = acts assignment operator (similar = in c). i.e., sets left operand (a column name) value of right operand. legal statement - declaring not know value of column.


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