Revision
Comparisons with NULL
Functions with NULL
COALESCE to the rescue
NULLIF
Revision and practice
23. Summary

Instruction

Very nice. It's time to review what we've learned about NULLs so far:

  • Rule number one: never trust a NULL. Keep your eyes open.
  • Use the operator IS NULL to check if the column is NULL.
  • Use the operator IS NOT NULL to check if the column is not NULL.
  • The equality and non-equality conditions (price = 7, price = NULL, price > 15) are NEVER true when the argument is NULL.
  • Arithmetical operations (e.g. a + b) and most functions (e.g. a || b, length(a)) will return a NULL if any of the values is a NULL.
  • COALESCE(x,y,…) returns the first non-NULL argument.
  • NULLIF(x,y) returns x when x != y or NULL when
    x = y.
  • In some databases, you can use NULLS FIRST or NULLS LAST to specify how NULLs are treated in the ORDER BY clause.

    Are you ready for a short quiz?

Exercise

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Console

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Result

TableConsole