Outer Joins
Inner v.s. Outer Joins
| Type | Desc |
|---|---|
INNER JOIN |
仅返回符合条件的行 |
OUTER JOIN |
返回所有的行，若不符合条件，返回null |
Left v.s. Right Outer Joins
| Type | Desc |
|---|---|
LEFT OUTER JOIN |
以JOIN左边的行数为准 |
RIGHT OUTER JOIN |
以JOIN右边的行数为准 |
Three-Way Outer Joins
SELECT $tbl.$fld...
FROM $tbl_name
LEFT JOIN $tbl_name2
ON $condition1
LEFT JOIN $tbl_name3
ON $condition2