Learned:
NULL semantics are unclear
Query is not easy to optimize
If it represents a null value, there can be other alternatives (logically)
VARCHAR(100) NOT NULL DEFAULT '';
INT NOT NULL DEFAULT 0;
I hope to understand the attitude towards NULL when designing a scheme (I remember reading an article saying that you should try: NOT NULL DEFAULT XXX)
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Yes, NULL columns should be avoided as much as possible, and default values should be explicitly set as much as possible, especially for indexed columns. In mysql, null takes up space. If the value is indexed, its index is invalid.
Yes, when judging that a field is not empty, you also need to use is not null. NULL in mysql actually takes up space. NULL values will not be stored in B-tree indexes, so if the indexed field can be NULL, the index The efficiency will drop a lot.