QUERY: SELECT 1 AS one WHERE 1 IN (SELECT 1);
one
---
  1
(1 row)

QUERY: SELECT 1 AS zero WHERE 1 NOT IN (SELECT 1);
zero
----
(0 rows)

QUERY: SELECT 1 AS zero WHERE 1 IN (SELECT 2);
zero
----
(0 rows)

QUERY: CREATE TABLE SUBSELECT_TBL (
  f1 integer,
  f2 integer,
  f3 float
);
QUERY: INSERT INTO SUBSELECT_TBL VALUES (1, 2, 3);
QUERY: INSERT INTO SUBSELECT_TBL VALUES (2, 3, 4);
QUERY: INSERT INTO SUBSELECT_TBL VALUES (3, 4, 5);
QUERY: INSERT INTO SUBSELECT_TBL VALUES (1, 1, 1);
QUERY: INSERT INTO SUBSELECT_TBL VALUES (2, 2, 2);
QUERY: INSERT INTO SUBSELECT_TBL VALUES (3, 3, 3);
QUERY: INSERT INTO SUBSELECT_TBL VALUES (6, 7, 8);
QUERY: INSERT INTO SUBSELECT_TBL VALUES (8, 9, NULL);
QUERY: SELECT '' AS eight, * FROM SUBSELECT_TBL;
eight|f1|f2|f3
-----+--+--+--
     | 1| 2| 3
     | 2| 3| 4
     | 3| 4| 5
     | 1| 1| 1
     | 2| 2| 2
     | 3| 3| 3
     | 6| 7| 8
     | 8| 9|  
(8 rows)

QUERY: SELECT '' AS two, f1 AS "Constant Select" FROM SUBSELECT_TBL
  WHERE f1 IN (SELECT 1);
two|Constant Select
---+---------------
   |              1
   |              1
(2 rows)

QUERY: SELECT '' AS six, f1 AS "Uncorrelated Field" FROM SUBSELECT_TBL
  WHERE f1 IN (SELECT f2 FROM SUBSELECT_TBL);
six|Uncorrelated Field
---+------------------
   |                 1
   |                 2
   |                 3
   |                 1
   |                 2
   |                 3
(6 rows)

QUERY: SELECT '' AS six, f1 AS "Uncorrelated Field" FROM SUBSELECT_TBL
  WHERE f1 IN (SELECT f2 FROM SUBSELECT_TBL WHERE
    f2 IN (SELECT f1 FROM SUBSELECT_TBL));
six|Uncorrelated Field
---+------------------
   |                 1
   |                 2
   |                 3
   |                 1
   |                 2
   |                 3
(6 rows)

QUERY: SELECT '' AS six, f1 AS "Correlated Field", f3 AS "Second Field"
  FROM SUBSELECT_TBL
  WHERE f1 IN (SELECT f2 FROM SUBSELECT_TBL WHERE f2 = f1);
six|Correlated Field|Second Field
---+----------------+------------
   |               1|           3
   |               2|           4
   |               3|           5
   |               1|           1
   |               2|           2
   |               3|           3
(6 rows)

QUERY: SELECT '' AS six, f1 AS "Correlated Field", f3 AS "Second Field"
  FROM SUBSELECT_TBL
  WHERE f1 IN (SELECT f2 FROM SUBSELECT_TBL WHERE CAST(f2 AS float) = f3);
six|Correlated Field|Second Field
---+----------------+------------
   |               1|           3
   |               2|           4
   |               3|           5
   |               1|           1
   |               2|           2
   |               3|           3
(6 rows)

QUERY: SELECT '' AS six, f1 AS "Correlated Field", f3 AS "Second Field"
  FROM SUBSELECT_TBL
  WHERE f1 IN (SELECT f2 FROM SUBSELECT_TBL WHERE f2 = CAST(f3 AS integer));
ERROR:  dtoi4: unable to convert null
QUERY: SELECT '' AS five, f1 AS "Correlated Field"
  FROM SUBSELECT_TBL
  WHERE (f1, f2) IN (SELECT f2, CAST(f3 AS int4) FROM SUBSELECT_TBL WHERE f3 IS NOT NULL);
five|Correlated Field
----+----------------
    |               2
    |               3
    |               1
    |               2
    |               3
(5 rows)

QUERY: SELECT '' AS three, f1 AS "Correlated Field"
  FROM SUBSELECT_TBL
  WHERE (f1, f2) NOT IN (SELECT f2, CAST(f3 AS int4) FROM SUBSELECT_TBL WHERE f3 IS NOT NULL);
three|Correlated Field
-----+----------------
     |               1
     |               6
     |               8
(3 rows)

QUERY: SELECT '' AS eight, ss.f1 AS "Correlated Field", ss.f3 AS "Second Field"
  FROM SUBSELECT_TBL ss
  WHERE f1 NOT IN (SELECT f1 FROM INT4_TBL WHERE f1 != ss.f1);
eight|Correlated Field|Second Field
-----+----------------+------------
     |               1|           3
     |               2|           4
     |               3|           5
     |               1|           1
     |               2|           2
     |               3|           3
     |               6|           8
     |               8|            
(8 rows)