# foos TABLE
#| id | name                |
#| 1  | No associated bars  |
#| 2  | bar WHERE col = 'x' |
#| 3  | bar WHERE col = 'y' |

# bars TABLE
#| id | foo_id | col |
#| 1  | 2      | 'x' |
#| 2  | 3      | 'y' |

# ALL results FROM the OUTER JOIN
SELECT * FROM foos LEFT OUTER JOIN bars ON foos.id = bars.foo_id
#| foos.id | foos.name           | bars.id | bars.foo_id | col  |
#| 1       | No associated bars  | NULL    | NULL        | NULL |
#| 2       | bar WHERE col = 'x' | 1       | 2           | 'x'  |
#| 3       | bar WHERE col = 'y' | 2       | 3           | 'y'  |

# The query the OP AND myself would LIKE TO be executed
SELECT * FROM foos LEFT OUTER JOIN bars ON foos.id = bars.foo_id AND bars.col = 'x'
#| foos.id | foos.name           | bars.id | bars.foo_id | col  |
#| 1       | No associated bars  | NULL    | NULL        | NULL |
#| 2       | bar WHERE col = 'x' | 1       | 2           | 'x'  |
#| 3       | bar WHERE col = 'y' | NULL    | NULL        | NULL |

# Your (close but wrong) answer
SELECT * FROM foos LEFT OUTER JOIN bars ON foos.id = bars.foo_id WHERE bars.col = 'x'  OR bars.id IS NULL
#| foos.id | foos.name           | bars.id | bars.foo_id | col  |
#| 1       | No associated bars  | NULL    | NULL        | NULL |
#| 2       | bar WHERE col = 'x' | 1       | 2           | 'x'  |

#The 3rd ROW would have bars.col = 'y' AND bars.id = 2, so it won't return the row. We need the row to return the foo.