CASE函数经常看到，NULLIF不怎么用到。今天学习一下。

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NULLIF
Syntax
Purpose

NULLIF compares expr1 and expr2. If they are equal, then the function returns null. If
they are not equal, then the function returns expr1. You cannot specify the literal NULL
for expr1.
If both arguments are numeric data types, then Oracle Database determines the
argument with the higher numeric precedence, implicitly converts the other argument
to that data type, and returns that data type. If the arguments are not numeric, then
they must be of the same data type, or Oracle returns an error.
The NULLIF function is logically equivalent to the following CASE expression:
CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END
Examples
The following example selects those employees from the sample schema hr who have
changed jobs since they were hired, as indicated by a job_id in the job_history table
different from the current job_id in the employees table:
SELECT e.last_name, NULLIF(j.job_id, e.job_id) "Old Job ID"
FROM employees e, job_history j
WHERE e.employee_id = j.employee_id
ORDER BY last_name, "Old Job ID";
LAST_NAME Old Job ID
------------------------- ----------
De Haan IT_PROG
Hartstein MK_REP
Kaufling ST_CLERK
Kochhar AC_ACCOUNT
Kochhar AC_MGR
Raphaely ST_CLERK
Taylor SA_MAN
Taylor
Whalen AC_ACCOUNT
Whalen