Solve the problem that MySQL read-write separation causes data not to be selected after insert

MySQL sets up independent writing separation. If you write the following in the code, problems may occur

//Enter first this.insert(obj); 
// Query again Object res = this.selectById(obj.getId());
res: null;

A pitfall online: after doing read-write separation, there is a scenario where you want to reuse the method and only pass in an ID. You can directly find an object in the database for subsequent processing. As a result, you can't find it. Various transaction isolation levels have been checked. Finally, it is found that it is a problem of read-write separation, so change the idea to implement it.

Additional knowledge: MySQL INSERT insertion condition judgment: insert if it does not exist

We often need to perform batch inserts in SQL, requiring: if the record does not exist, insert it; if it does exist, do not insert it. How about using an INSERT statement?

For ordinary INSERT insertion, if we want to ensure that no duplicate records are inserted, we can only create a unique constraint for a certain field;

Is there a solution that does not create a unique constraint and can be achieved with just one INSERT INTO statement?

Answer: Use INSERT INTO IF EXISTS. The specific syntax is as follows

INSERT INTO table(field1, field2, fieldn) 
SELECT 'field1', 'field2', 'fieldn' 
FROM DUAL
WHERE NOT EXISTS(SELECT field FROM table WHERE field = ?)

example:

INSERT INTO a (order_id, operator, oper_date, memo) 
SELECT '3', 'onion3', '2017-11-28', 'Test 3' 
from DUAL 
where not exists(select order_id from a where operator='onion3' and memo = '测试3');

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