Processing of null value of Oracle notes

Null value null The reason for this

Oracle Null value in database query null The situation of is generally composed of the following two situations ：
1, The data table design is not perfect , For example, there is no foreign key constraint 、 No default value specified 、 Not specified, cannot be empty, etc .
2, Use left join、right join perhaps full join Inquire about , Data fields that are not associated will use null Value padding .

Null value null Hazards of

1, Breaking the integrity of data 、 Readability .
2, influence sql Elegance and performance of statements .
3, Impact on index performance .
4, In particular, it affects numerical calculation .

Null value null To deal with

The best way is to solve the problem once and for all in terms of database design and business data processing logic , But in reality, this is very difficult to do , Here are a few simple examples null Worth the function .

nvl function

Grammar format ：nvl( expression , The default value is )
The most commonly used null value processing function ,nvl First, judge whether the expression is empty , If it is null null Then the default value is used to replace , Not null, return the original value of the expression .
Such as select nvl(null,'123') from dual
The result of the query is ’123’, Statement means to use string ’123’ Instead of null value .

decode function

Grammar format ：decode( expression , Conditions 1, result 1,··· Conditions n, result n, The default value is )
The expression satisfies the condition 1 Return results 1,··· Meet the conditions n Return results n, If they are not satisfied, the default value is returned .
Be careful ： If no default value is specified , Then return to control null.
Such as ：select decode(sign(x-y),0,' zero ',1,' just ',' negative ') from dual
Judge x-y The symbol of , If 0 return ‘‘ zero ’’, If 1 return ‘ just ’, Other situations return to ‘ negative ’（ contain x,y At least one is null null The situation of ）.

coalesce function

Grammar format ：coalesce( expression 1,···· expression n, The default value is )
Judge the expression in turn 1,··· expression n, Returns the first non null value null The expression of , If the expression 1,··· expression n If both are empty, the default value is returned .
It is often used when the field value is retrieved by priority in association query .
Such as ：

Select coalesce(a.orderid, b.name, c.name, d.name, ' Not obtained ')
  From so a
  Left Join members_hot b
    On a.memberid = b.id
  Left Join members c
    On a.memberid = c.id
  Left Join members_cold id
    On a.memberid = d.id
 Where create_time >= trunc(Sysdate, 'year')

Use the order form to associate with the member form to obtain the member name , The priority for members_hot ,members,members_cold, If there is no match, it returns ‘ Not obtained ’.