1.2 Table Analysis
select * form users left join dept on users.deptId = dept.id
Driven by the left side, the data of the left table is naturally available, so the id index needs to be established in the dept table. This is a special case, because the department table id is the primary key, and the index is automatically established.
2.3 Table Analysis
select * form users left join dept on users.deptId = dept.id left join wages on user.id = wages.user_id
At this point, you need to create an index in the department table id and the wages table user_id
Summary: left join to the right table to create an index, right join you know.