[MySQL] sql99 syntax to realize multi table query

1. SQL92： Use (+) Create connection

• stay SQL92 Used in (+) Represents the location of the slave table . namely Left or right outer connection in ,(+) Indicates which is from the table .

• Oracle Yes SQL92 Better support , and MySQL Do not support SQL92 External connection of .

• Oracle Realization

# The left outer join 
SELECT last_name,department_name
FROM employees,departments
WHERE employees.department_id = departments.department_id(+);

# Right connection 
SELECT last_name,department_name
FROM employees ,departments
WHERE employees.department_id(+) = departments.department_id;

• SQL92 Syntax to implement inner connection ： as follows
  practice ： Query all employees last_name,department_name Information

SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.department_id;   

• And in SQL92 in , Only the left outer connection and the right outer connection , Not full （ Or all ） External connection .

2. SQL99 Syntax to implement multi table query

• Use JOIN...ON Clause creates the syntax structure of the join ：
• SQL99 Use... In grammar JOIN ...ON To realize the query of multiple tables . This method can also solve the problem of external connection .MySQL It supports this way .

  SELECT table1.column, table2.column,table3.column
  FROM table1
      JOIN table2 ON table1  and  table2  The connection condition of 
          JOIN table3 ON table2  and  table3  The connection condition of 

• SQL99 Syntax to implement inner connection ：

SELECT last_name,department_name
FROM employees e INNER JOIN departments d
ON e.`department_id` = d.`department_id`;

SELECT last_name,department_name,city
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`
JOIN locations l
ON d.`location_id` = l.`location_id`;

• SQL99 Syntax implementation external connection ：

practice ： Query all employees last_name,department_name Information

• The left outer join ：

SELECT last_name,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`;

• Right connection ：

SELECT last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;

• Full outer join ：mysql I won't support it FULL OUTER JOIN

SELECT last_name,department_name
FROM employees e FULL OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;

• SQL99 This nested structure is very refreshing 、 More hierarchical 、 More readable , Even if more tables are connected, they are clearly visible . If you take SQL92, Readability will be greatly reduced .