操作符
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NULL-safe equal. This operator performs an equality comparison like the = operator, but returns 1 rather than NULL if both operands are NULL, and 0 rather than NULL if one operand is NULL.
NULL安全的相等比较操作符。这个操作符和‘=’操作符一样用来执行相等的比较,但返回1而不是NULL如果两个操作数都是NULL,返回0而不是NULL如果只有一个操作数是零。NULL-safe equal. This operator perf
