如何在将记录插入数据库表Java中之前检查非空字段

Question

I have to create / insert a new record in a database table (MySQL, Oracle, MSSQL) only by inserting the primary key field values. But this is not working if there is a column in the table which does not allow null values. How can I check the table columns which do not allow null values.

Answer 1

You can execute a request against your table, for example at startup time, and then check the metadata of your table:

ResultSet rs = stmt.executeQuery("SELECT a, b, c FROM myTable");
ResultSetMetaData rsmd = rs.getMetaData();
int nullable = rsmd.isNullable(1);
if(nullable == ResultSetMetadata.columnNullable) {
    System.out.println("Nullable");
}

Answer 2

Check out the DatabaseMetadata object and in particular the getAttributes() method.

NULLABLE int => whether NULL is allowed

 attributeNoNulls - might not allow NULL values attributeNullable - definitely allows NULL values attributeNullableUnknown - nullability unknown 

I do think (however) that your application should know as much as possible re. the data constraints. The database protects itself with these constraints, but the application should (where possible) know and respect this info.

Answer 3

It looks like this should work: ResultSetMetaData.isNullable

public int isNullable(int column)
           throws SQLException

Indicates the nullability of values in the designated column.

Parameters:
    column - the first column is 1, the second is 2, ... 
Returns:
    the nullability status of the given column; one of columnNoNulls, columnNullableor columnNullableUnknown 
Throws:
    SQLException - if a database access error occurs

Answer 4

If I get you right, you are looking for a way to test the tables in your program (and I assume not manually)...
Test each table like this:
Step 1: Try inserting a record with null values to your table.
Step 2: If exception rises, catch it and proceed these steps with other tables.

You can mark those tables or have a list which contains tables' names for you.

Answer 5

You could check ResultSetMetaData as well. javadoc

ResultSetMetaData.columnNullable
ResultSetMetaData.columnNoNulls
ResultSetMetaData.columnNullableUnknown

Regards