Multiple table query
Case description
Understanding of Cartesian product
select id,department_name from employees,departments;#错的 select id,department_id,department_name from employees CROSS JOIN departments;#错的
Each employee and each department were matched once (number of entries found = Number of ids * number of departments)
Cause of error: missing connection conditions
Solution to Cartesian product
Write connection conditions: Table 1. Column = Table 2. Column (if To connect multiple tables, at least n-1 connection conditions must be used)
select id,employees.name,department_name from employees,departments WHERE employees.name = departments.name;
Note: If the column to be displayed has the same name in the table to be queried, it must be indicated which table it comes from, eg: employees .name
It is recommended to indicate which table information is displayed when querying multiple tables (optimization)
Optimization: You can use the alias of the table after FROM, but once the alias is used, subsequent Be sure to use aliases
Classification of multi-table queries
Equivalent joins and non-equivalent joins
Equivalent joins: The above ones with =
- ##Non-equijoin: No=
select t1.id,t1.name,t2.grade from employees t1,departments t2 WHERE ti.salary BETWEEN t2.lowest_salary AND t2.highest_salary ;#非等值Self-join and non-self-join
- Non-self-link: Table 1 and Table 2 are connected
- Self-link: Table 1 and itself are connected
#显示员工(t1)和其管理者(t2)的基本信息 select t1.id,t1.name,t2.id,t2.name from employees t1,employees t2#一个表看作两个表 WHERE t1.manage_id = t2.id ;#自连接Inner connection and outer connection Connection
- Inner join: merge tables containing the same column, the result does not include rows that do not match one table with another table
- Except Join: Merge tables containing the same column. In addition to the results of the inner join, unmatched rows are also queried
WHERE t1.department_id = t2.department_id(+)#左连接SQL99: Use JOIN...ON methodinner join
select t1.id,t1.name,t2.department_name,t3.environment
from employees t1 JOIN departments t2
ON t1.department_id = t2.department_id
JOIN locations t3#加入第二个人表
ON t2.department_location = t3.department_location;
OUTER JOINUse OUTER JOIN...ON...
- Left outer join: LEFT OUTER JOIN
- RIGHT OUTER JOIN
- Full OUTER JOIN: FULL OUTER JOIN (MySQL does not support)
select t1.name,t2.department_name#左外连接 from employees t1 LEFT OUTER(可省略) JOIN departments t2 ON t1.department_id = t2.department_id;The use of UMIONMerge query results
SELECT colum... FROM table1 UNION (ALL) SELECT colum... FROM table2
- UNION operator
- UNION ALL operator (recommended)
select t1.name,t2.department_name from employees t1 JOIN departments t2 ON t1.department_id = t2.department_id;Upper left picture (left outer join):
select t1.name,t2.department_name from employees t1 LEFT JOIN departments t2 ON t1.department_id = t2.department_id;Top right picture (right outer join):
select t1.name,t2.department_name from employees t1 RIGHT JOIN departments t2 ON t1.department_id = t2.department_id;Middle left picture:
select t1.name,t2.department_name from employees t1 LEFT JOIN departments t2 ON t1.department_id = t2.department_id WHERE t2.department_id IS NULL;Middle right picture:
select t1.name,t2.department_name from employees t1 RIGHT JOIN departments t2 ON t1.department_id = t2.department_id WHERE t1.department_id IS NULL;Bottom left picture (full outer join) :
#方式一:左上图 UNION ALL 右中图 select t1.name,t2.department_name from employees t1 LEFT JOIN departments t2 ON t1.department_id = t2.department_id UNION ALL select t1.name,t2.department_name from employees t1 RIGHT JOIN departments t2 ON t1.department_id = t2.department_id WHERE t1.department_id IS NULL; #方式二:左中图 UNION ALL 右上图 select t1.name,t2.department_name from employees t1 LEFT JOIN departments t2 ON t1.department_id = t2.department_id WHERE t2.department_id IS NULL UNION ALL select t1.name,t2.department_name from employees t1 RIGHT JOIN departments t2 ON t1.department_id = t2.department_id;Bottom right picture:
#左中图 UNION ALL 右中图 select t1.name,t2.department_name from employees t1 LEFT JOIN departments t2 ON t1.department_id = t2.department_id WHERE t2.department_id IS NULL UNION ALL select t1.name,t2.department_name from employees t1 RIGHT JOIN departments t2 ON t1.department_id = t2.department_id WHERE t1.department_id IS NULL;New features of SQL syntaxNatural connectionUse keywords: NATURAL JOIN (inflexible), automatic query All the same fields in the table, and then perform an equal value connection USING connection (not applicable to self-join)Use the keyword: USING (field with the same name) to automatically join the fields with the same name in the table Equijoining
select t1.name,t2.department_name from employees t1 JOIN departments t2 ON t1.department_id = t2.department_id; 等价于 select t1.name,t2.department_name from employees t1 JOIN departments t2 USING(department_id);
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