MySQL：如果表2中表1中的记录计数为0或1，则将其添加到表3中

Question

Basically I'm trying to build a table (we'll call it table3) that has only the records from table1 that have counts of 0, 1, or do not appear at all in table2. Table1 and table3 are in database1 and table2 is in database2.

Here is the closest I can get to getting this working.

INSERT INTO database1.table3 (col1, col2, ...)
SELECT t1.col1, t1.col2, ...
FROM database1.table1 AS t1
LEFT JOIN database2.table2 AS t2 ON t1.columnToMatch = t2.columnToMatch
WHERE (t2.count < 2);

TLDR: I want everything from table1 that doesn't appear in table2 to be added to table3. If the record in table1 appears in table2 I only want to include it in table3 if table2.count is 0 or 1.

Thank you

Answer 1

Welcome to SO.

You are looking for an LEFT EXCLUDING JOIN - see here

To get all records from table1 that doesn't appear in table2 do a

SELECT table1.* 
FROM table1 
LEFT JOIN table2
ON table1.columnToMatch = table2.columnToMatch 
WHERE table2.columnToMatch IS NULL

After that get all matchting records with Count 0 or 1:

SELECT table1.*
FROM database1.table1 AS t1
INNER JOIN database2.table2
ON table1.columnToMatch = table2.columnToMatch
WHERE table2.count IN (0, 1)

Then you can append both results with a UNION ALL and store the whole bunch in a new table:

CREATE TABLE table3
SELECT ... FROM ...
UNION ALL
SELECT ... FROM ...