Question

This is my table:

ID      COUNTRY_CODE_3D     COUNTRY_CODE_2D      TYPE_ID    
 1          "IND"            "IN"                  11           
 2          "CAN"            "CA"                  13           
 3           null            null                  17

I want a select query which will return a single row 
if COUNTRY_CODE_2D = 'CA' (valid data) return 2nd row, else return the 3rd row where COUNTRY_CODE_2D =null and TYPE_ID=17

Answer 1

If you can put a functional index on country_code_2d, (NVL(country_code_2d,'EMPTY') )

CREATE INDEX fidx_empty_null ON country_cnfg_tbl (NVL(country_code_2d,'EMPTY'));

,you can then

SELECT * FROM country_cnfg_tbl a
WHERE NVL(a.country_code_2d,'EMPTY') IN 
(  
    SELECT NVL(b.country_code_2d,'EMPTY') FROM dual d
    LEFT JOIN country_cnfg_tbl b on b.country_code_2d = 'CA'
)

Or Without joins (and more portable as it does not use the oracle table DUAL)

SELECT * FROM country_cnfg_tbl a WHERE NVL(a.country_code_2d,'EMPTY') IN  
(       
    SELECT CASE WHEN count(*) = 0 THEN 
                'EMPTY' 
           ELSE 
                b.country_code_2d 
           END as country_code_2d  
    FROM country_cnfg_tbl b 
    WHERE b.country_code_2d = 'CA'
    GROUP BY b.country_code_2d
)