从表 B 更新表 A 中的列，其中值不存在于 MYSQL 中表 B 的不同结果中

Question

I have two tables both containing a column called domain .

Table A

|id | domain        |
|-------------------|
| 1 | google.com    |
| 2 | google.com    |
| 3 | yahoo.com     |
| 4 | microsoft.com |
| 5 | microsoft.com |
| 6 | slack.com     |
| 7 | loom.com      |
| 8 | loom.com      |

Table B

|id | domain        |
|-------------------|
| 1 | google.com    |
| 2 | yahoo.com     |
| 3 | microsoft.com |

I want to add rows to Table B for values in Table A that:

1. Don't exist in Table B
2. Are distinct in Tabel A

So in the use case above, Table B would end up like this:

|id | domain        |
|-------------------|
| 1 | google.com    |
| 2 | yahoo.com     |
| 3 | microsoft.com |
| 4 | slack.com     |
| 5 | loom.com      |

Here is where I'm at with the query, which doesn't work obviously:

UPDATE tableb tb SET tb.domain = ta.domain WHERE tb.domain NOT IN (SELECT DISTINCT(domain) FROM tablea) ta LIMIT 1

Answer 1

Use exists logic:

INSERT INTO TableB (domain)   -- assuming id is auto increment
SELECT DISTINCT domain
FROM TableA a
WHERE NOT EXISTS (SELECT 1 FROM Tableb b WHERE b.domain = a.domain);

The exists clause ensures that the domain from A table does not already exist in the B table. We select distinct domains from the A table to avoid inserting the same domain more than once.