Join query is the most important query in relational data, and can query two or more tables.
Type of connection:
Inner joins: use symbolic joins like "=, >, <"
Outer join: only limit to one table
Left outer join: no restrictions on the table on the left side of the join
Right outer join: no restrictions on the table on the right side of the join
Full outer join: no restrictions on both tables
Cross join: Perform Cartesian set operations on two tables to form a new result table, the number of rows in the result table is equal to the product of the number of rows in the two tables
here comes the point
First, the realization of the connection query
1. Use the FROM clause and the WHERE clause to implement multi-table queries
(1). Use the FROM clause to directly implement the connection of the table
SELECT [student].s_id,[Student].s_name,[Grade].c_grade FROM [Student],[Grade] ORDER BY [Student].s_id
(2). Use the WHERE clause to achieve conditional joins
SELECT [student].s_id,[Student].s_name,[Grade].c_grade FROM [Student],[Grade] WHERE [Student].s_id = [Grade].s_id
(3). Use the WHERE clause to achieve self-connection
SELECT g.* FROM [Student] s ,[Grade] g WHERE s.s_name = '张三' AND s.s_id = g.s_id
2. Use the JOIN keyword to implement multi-table query
(1). Use [INSERT]JOIN to implement inner join query
SELECT [Student].s_id,[Student].s_name,[Grade].c_grade
FROM [Student] INNER JOIN [Grade]
ON [Student].s_id = [Grade].s_id
(2). Use LEFT[OUTER]JOIN to implement left outer join query
SELECT [Student].s_id,s_name,c_grade
FROM [Student] LEFT OUTER JOIN [Grade]
ON [Student].s_id = [Grade].s_id
(3). Use RIGHT[OUTER]JOIN to implement right outer join query
SELECT [Grade].c_id,[Grade].s_id,c_grade
FROM [Student] RIGHT OUTER JOIN [Grade]
ON [Student].s_id = [Grade].s_id
(4). Use FULL[OUTER]JOIN to achieve full outer join query
(5). Use CROSS JOIN to implement cross join query