Join to a table only when column value is true

select * from T1
join T2 on T1.value = T2.value
where T2.value2 = 'variable2'
and T2.value3 = 'variable3'

select *
from T1
join T2
  on  T1.value = T2.value
  and T1.booleancolumn = 1
where T2.value2 = 'variable2'
and T2.value3 = 'variable3';

SELECT
    COALESCE(T2.Value, T1.Value) AS Value,
    COALESCE(T2.Value2, T1.Value2) AS Value2,
    COALESCE(T2.Value3, T1.Value3) AS Value3
FROM T1
LEFT JOIN T2 ON T2.value = T1.value
    AND T2.Value2= @Variable2 AND T2.Value3 = @Variable3

select * from T1
join T2 on T1.value = T2.value
where T2.value2 = case T1.useT2 when 'yes' then 'variable2' else T2.value2 END
and T2.value3 = case T1.useT2 when 'yes' then 'variable3' else T2.value3 END

SELECT * FROM T1
INNER JOIN T2
  ON T1.Value = T2.Value
WHERE T2.Value2 = 'variable2'
AND T2.Value3 = 'variable3'
AND T1.booleancolumn = 1

SELECT T1.*
  FROM T1
  WHERE T1.use_t2 = FALSE
  AND T1.value2 = 'variable2'
  AND T1.value3 = 'variable3'
UNION ALL
SELECT T2.*
  FROM T1 JOIN T2 ON T1.value = T2.value
  WHERE T1.use_t2 = TRUE
  AND T2.value2 = 'variable2'
  AND T2.value3 = 'variable3'