Question

I have to update a column in Table ‘A’ with values from Table ‘B’. If any value in Table ‘B’
is null or empty then I have to get the value from table ‘C’.

Manu

Answer 1

Basically:

UPDATE a SET a.FIELD = (CASE WHEN b.FIELD IS NULL or b.FIELD = '' THEN c.FIELD ELSE b.FIELD END)
FROM TABLEA a 
LEFT JOIN TABLEB b on a.id = b.someid
LEFT JOIN TABLEC c on a.id = c.someid

Joins may or may not be LEFT, depending on your data, and you may want to handle the case where both b.field and c.field are null.