SQL multi-Table query statement set

Today, let's talk about the SQL statements on how to query multiple tables. First, the most common and abrupt multi-Table query method is one.

Select * from A join B on a. uid = B. uid join C ON C. uid = A. uid where uid = 1

2. Four table queries

Select * form A, B, C, D where A. uid = B. uid and B. uid = C. uid and C. uid = D. uid where uid = 1

Iii. query by alias

Select * from table1 as a, table2 as B where a. id = B. id and a. id = 2

4. left join Table query

Select * from table1 a left join table2 B where a. id = B. id and a. id = 2

For the preceding four query statements, the most efficient is left join. If it is not necessary, never use multi-table join queries.