How to distinct result in SQL VIEW select query with JOIN?

Question

I have a view with select query with join.

SELECT
 t.a, 
 t.b,
 (SELECT CASE WHEN t2.field IS NULL THEN 0 ELSE 1 END AS Expr1) AS C
FROM table1 t1
LEFT JOIN table2 t2 on t2.link = t1.link

If in table2 there is more than 1 entry with same link, i'm getting duplicated entries.

What is the correct way to distinct result? Is there a type of JOIN, that won't return all result, but only single row?

Thank you

Answer 1

select distinct t.a, t.b ...

应该消除重复。

Answer 2

If they are really duplicates, if you use distinct they are removed:

SELECT
 distinct
 t.a, 
 t.b,
 (SELECT CASE WHEN t2.field IS NULL THEN 0 ELSE 1 END AS Expr1) AS C
FROM table1 t1
LEFT JOIN table2 t2 on t2.link = t1.link

Answer 3

SELECT
    t.a, 
    t.b,
    CASE WHEN EXISTS (SELECT * FROM table2 t2 WHERE t2.link = t1.link) 
           THEN 1 
           ELSE 0
    END AS c
FROM table1 t1