Pit avoidance: description of null values in in and not in SQL

Catalog

Sql in in and not in There is null Situation of value

1.not in Contained in the null Situation of value

2.in Contained in the null Situation of value

mysql There is a default value but it is still inserted null problem

Sql in in and not in There is null Situation of value

1)in The logical rule of is or not in The logical rule of is and

2) Judge null Of sql Statement for is not null perhaps is null

3) When you meet null = null Your judgment is due to the inconformity null The rules of judgment , So the result must be flase

1.not in Contained in the null Situation of value select  *  from A  where  A.name not  in     (select B.name          from B )

Above not in If B Tabular name When the field has a null value , In any case, the query result of the above statement must be empty

2.in Contained in the null Situation of value select  *  from A  where  A.name in     (select B.name          from B )

Above in If B Tabular name When the field has a null value , If A Presence in table name The field also has a null value , Then the query result of the above statement will not display A Null value in table

mysql There is a default value but it is still inserted null problem

Today, I found that the value of a column of a corresponding record in the project is null, But the value of this column for this record should not be null.

Check the code and find that there is no assignment to the member variable corresponding to this column when inserting , Even if the database column has a default value , When inserting into the database, it is still inserted null.

mysql The default value of is only in insert It will only take effect if there is no such field in the statement , If insert This field is inserted in the, and the value of this field is null,null Values will be inserted into the table , The default value is now invalid .

stay mybatis In development, a table object is usually inserted directly , All fields in the table will be inserted , So if you want to set the default value , Logic should be written at the business level , Instead of relying on mysql Of default To set the default value .

In addition, try to ensure that the new database should use dynamic sql To operate , This can also avoid the above problems .

The above is personal experience , I hope I can give you a reference , I also hope you can support the software development network .