Sql query for updating database if value is not nu

问题:

I am having a table which has about 17 fields. I need to perform frequent updates in this table. But the issue is each time I may be updating only a few fields. Whats the best way to write a query for updating in such a scenario? I am looking for an option in which the value gets updated only if it is not null.

For example I have four fields in database Say A,B,C,D. User updates the value of say D. All other values remains the same. So I want an update query which updates only the value of D keeping the others unaltered. SO if i put a,b and c as null and d with the value supplied by user I want to write an update query which only updates the value of d as a,b and c is null. Is it something achievable?

I am using SQLite database.

Could someone please throw some light into it?

回答1:

Without knowing your database it's tough to be specific. In SQL Server the syntax would be something like ...

UPDATE MyTable 
SET 
        Field1 = IsNull(@Field1, Field1),
        Field2 = IsNull(@Field2, Field2),
        Field3 = IsNull(@Field3, Field3)
WHERE 
     <your criteria here>

EDIT

Since you specified SQLLite ...replace my IsNull function with COALESCE() or alternately look at the IfNull function.

回答2:

 UPDATE your_table
    SET some_column = 42
 WHERE some_column IS NOT NULL

Will only update some_column if it's not null

回答3:

Posting a SQL Server solution with 2 tables for posterity. Query joins two tables and updates the values that are present. Otherwise original value is maintained.

tables = table1, table2 each having field1 and field2

update t1 WITH (ROWLOCK)
set T1.Field2 = ISNULL(T2.Field2,T1.Field2)
from Table1 T1 Join Table2 T2 
    ON T1.Field1 = T2.Field1