如何从表中获取只有在连接两个表之后没有值的结果

Question

Statement st=conn.createStatement();
String sql="SELECT a.ID,a.A_TEXT,a.B_TEXT,a.C,a.D_TO_E,a.F_TEXT,a.G,a.INTIME,b.EXIT_TIME FROM tm_A a LEFT JOIN tm_B b ON a.ID=b.ID WHERE EXIT_TIME=''";

ResultSet rs=st.executeQuery(sql);

while(rs.next())
    {

    arls.add(rs.getString("ID"));
    arls.add(rs.getString("A_TEXT"));
    arls.add(rs.getString("B_TEXT"));
    arls.add(rs.getString("C"));
    arls.add(rs.getString("D_TO_E"));
    arls.add(rs.getString("F_TEXT"));
    arls.add(rs.getString("G"));
    arls.add(rs.getString("INTIME"));
    arls.add(rs.getString("EXIT_TIME"));

    }

I have above code in that i want to retreive values from two tables where EXIT_TIME ='null' but i am getting nothing after executing though there are value in table tm_A but if i remove where clause then i am getting result and but under EXIT_TIME it displays 'null' and where there are values that are displaying

Answer 1

If you want to select a result where a column is null you have to use the is operator:

SELECT a.ID,a.A_TEXT,a.B_TEXT,a.C,a.D_TO_E,a.F_TEXT,a.G,a.INTIME,b.EXIT_TIME 
FROM tm_A a 
LEFT JOIN tm_B b ON a.ID=b.ID 
WHERE EXIT_TIME is null

Comparison with null will always result to unknown . That is why you need the is operator instead of =

using EXIT_TIME='' would compare EXIT_TIME to an empty string but not null

If you want all records that are not null in EXIT_TIME you can use

WHERE EXIT_TIME is not null

Edit

You can use case to avoid null in your textboxes:

    SELECT a.ID, 
       a.A_TEXT,
       a.B_TEXT,
       a.C,
       a.D_TO_E,
       a.F_TEXT,
       a.G,
       a.INTIME,
       case when b.EXIT_TIME is null
            then 'NA'
            else b.EXIT_TIME
       end as EXIT_TIME
FROM tm_A a 
LEFT JOIN tm_B b ON a.ID=b.ID 

Answer 2

Just say

WHERE EXIT_TIME is null

instead of

WHERE EXIT_TIME=''

Hope it helps you...