找到sql join失败的值

Question

说我有一个查询，例如：

SELECT *
FROM Table_1
JOIN Table_2
ON Table_1.Col_1 = Table_2.Col_1

所以我有100条记录，其中98条是相同的，所以查询将打印出98条中的98条。如何让SQL打印出未能满足连接的2？

Answer 1

使用LEFT JOIN：

SELECT *
FROM Table_1
LEFT JOIN Table_2 ON (Table_1.Col_1 = Table_2.Col_1)

如果Table_2子句不匹配，ON的字段将为NULL。然后，您就可以添加WHERE TABLE_2.Col_1 IS NULL，仅保留Table_1中Table_2内没有匹配项的记录。

Answer 2

LEFT JOIN的替代方法是使用EXISTS。

SELECT * FROM Table_1
WHERE NOT EXISTS (SELECT * FROM Table_2 WHERE Col_1 = Table_1.Col_1)

Answer 3

试试这个：

SELECT * FROM `Table_1` LEFT JOIN `Table_2` ON (`Table_1`.`Col_1` = `Table_2`.`Col_1`)

Answer 4

SELECT *
FROM Table1 AS A
LEFT OUTER JOIN Table2 AS B
ON (A.Col = B.Col)
WHERE B.Col IS NULL