问题
In Sybase and MSSqlServer TransactSQL, we have a function IsNull(columnName,valueForNull) to return a typed default value when a column value is null. How can I replicate this functionality in SQLite?
Example in TransactSQL:
select IsNull(MyColumn,-1) as MyColumn from MyTable
If MyColumn is null, this expression will return -1 as the value of MyColumn. Want to do something similar in SQLite.
(Yes, I know I can write my own wrapper function to handle this - that's not the answer I'm looking for)
TIA
回答1:
You can use ifnull:
select ifnull(MyColumn,-1) as MyColumn from MyTable
回答2:
You should use the standard COALESCE function:
select coalesce(MyColumn, -1) as MyColumn from MyTable
Any database that understands standard ANSI SQL will support COALESCE so using it is a good habit to get into.
回答3:
The SQL standard way to do this is with CASE:
SELECT CASE WHEN MyColumn IS NULL THEN -1 ELSE MyColumn END FROM MyTable
Considerably more verbose than engine-specific IFNULL, ISNULL, NZ functions, but more portable.
(Or, as mu points out, you can use the much better COALESCE for this).
回答4:
SQLite has the IFNULL() built-in function which would do what I think you're trying here.
来源：https://stackoverflow.com/questions/7999608/null-substitution-in-sqlite