Are null and empty string equivalent in Oracle?

In Oracle, NULL and empty string are not equal. NULL represents an unknown value and is not equal to any value; an empty string is a string of length 0 that can be equal to other empty strings. The difference between NULL and the empty string is that NULL cannot be used in arithmetic operations or concatenation operations, while the empty string can.

NULL and empty string are not equal in Oracle

In Oracle database, NULL and empty string ('') are two different values ​​that have different meanings when compared and processed.

NULL

• NULL represents an unknown or non-existent value.
• NULL has a special data type called the NULL type.
• NULL is not equal to any other value, including the empty string.

Empty string

• The empty string is a string of length 0 that does not contain any characters.
• The empty string has data type VARCHAR2 and has a length of 0.
• An empty string can be equal to other empty strings or strings of length 0.

Comparison Behavior

In Oracle, NULL and empty strings are compared using the following rules:

• NULL is compared with any Values ​​(including empty strings) are not equal.
• Empty strings can be equal to other empty strings.

For example:

<code class="sql">SELECT CASE
  WHEN 'abc' = NULL THEN 'NULL 等于空字符串'
  WHEN 'abc' = '' THEN '非 NULL 字符串等于空字符串'
  ELSE '不相等'
END FROM DUAL;</code>

Output:

<code>不相等</code>

Handling differences

The difference is in the treatment of NULL and empty characters String processing:

• NULL:

  • cannot be used for arithmetic operations.
  • will cause the comparison operation result to be NULL, even if compared with an empty string.
  • Cannot be concatenated with other strings using the concatenation operator (||).

• Empty string:

  • can be used for arithmetic operations, such as '0' '' = '0' .
  • During comparison operations, it is equal to other empty strings.
  • Can be concatenated with other strings using the concatenation operator.

Therefore, in Oracle, NULL and empty string are two different values ​​with different comparison behavior and processing rules. It is important to understand these differences to avoid Data processing error.

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