SQL table Connection Basic syntax

SQL connections can be divided into internal, external, and cross-joins.

1. Internal connection: Inner Join the query operation lists the data rows that match the join criteria, which uses comparison operators to compare the column values of the concatenated columns.

1.1 SELECT * from Table1 as a, Table2 as B where a.id= b.id

1.2 SELECT * FROM Table1 as a inner joins Table2 as B on a.id = b.ID

1. External connection
   2.1 Left OUTER join (simply put, left table data full display, right mismatch display null)

   SELECT * from Table1 as a left join Table2 as B on a.id = b.ID
   2.2 Right outer connection (opposite)

   SELECT * from Table1 as a right joins Table2 as B on a.id = b.ID

2.3 Full connection (when a row does not have a matching row in another table, the selection list column for the other table contains a null value.) If there are matching rows between the tables, the entire result set row contains the data values of the base table)

SELECT * from Table1 as a full outer join Table2 as B on a.id = b.ID

3. Each row in the left table is combined with all rows in the right table. Cross joins are also called Cartesian product

SELECT * from Table1 as a cross join Table2 as B Order b a.id

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SQL table Connection Basic syntax