MySQL Advanced (2) The execution sequence of a SQL statement Seven JOIN diagrams in SQL
1. SQL execution order
Human writing ideas:
1 SELECT DISTINCT
<select_list>
2 FROM
<left_table> <join_type>
3 JOIN <right_table> ON <join_condition>
4 WHERE
<where_condition>
5 GROUP BY
<group_by_list>
6 HAVING
<having_condition>
7 ORDER BY
<order_by_condition>
8 LIMIT <limit_number>
MySQL analysis ideas:
1 FROM <left_table>
2 ON <join_condition>
3 <join_type> JOIN <right_table>
4 WHERE <where_condition>
5 GROUP BY <group_by_list>
6 HAVING <having_condition>
7 SELECT
8 DISTINCT <select_list>
9 ORDER BY <order_by_condition>
10 LIMIT <limit_number>
Summary: SQL execution order
3. Seven kinds of JOIN in SQL
Seven kinds of Join Venn diagrams to help understand:
Test case:
Department table tbl_dept
CREATE TABLE tbl_dept (
id INT4 NOT NULL auto_increment,
dept_name VARCHAR ( 255 ) DEFAULT NULL,
ioc_add VARCHAR ( 255 ) DEFAULT NULL,
PRIMARY KEY ( id )
) ENGINE = INNODB AUTO_INCREMENT = 1 DEFAULT CHARSET = utf8;
Employee table tbl_emp
CREATE TABLE tbl_emp (
id INT4 NOT NULL auto_increment,
`name` VARCHAR ( 255 ) DEFAULT NULL,
dept_id INT4 DEFAULT NULL,
PRIMARY KEY ( id ),
KEY fk_dept_id ( dept_id )
#CONSTRAINT fk_dept_id FOREIGN KEY(dept_id) REFERENCES tbl_dept(id)
) ENGINE = INNODB AUTO_INCREMENT = 1 DEFAULT CHARSET = utf8;
Test Data
INSERT INTO tbl_dept ( dept_name, ioc_add ) VALUES ( 'RD', 11 );
INSERT INTO tbl_dept ( dept_name, ioc_add ) VALUES ( 'HR', 12 );
INSERT INTO tbl_dept ( dept_name, ioc_add ) VALUES ( 'MK', 13 );
INSERT INTO tbl_dept ( dept_name, ioc_add ) VALUES ( 'MIS', 14 );
INSERT INTO tbl_dept ( dept_name, ioc_add ) VALUES ( 'FD', 15 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e1', 1 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e2', 1 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e3', 1 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e4', 2 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e5', 2 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e6', 3 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e7', 4 );
INSERT INTO tbl_emp ( name, dept_id ) VALUES ( 'e8', 51 );
3.1 inner join
Inner join inner join query to obtain records in tables a and b that meet the on condition
SELECT
*
FROM
tbl_emp a
INNER JOIN tbl_dept b ON a.dept_id = b.id
3.2 Full jion
mysql does not support the full connection full jion writing method, it will report sql statement error
Solution: You can use the unoin keyword to merge the left and right connections and de-duplicate them.
SELECT
*
FROM
tbl_emp a
LEFT JOIN tbl_dept b ON a.dept_id = b.id
UNION
SELECT
*
FROM
tbl_emp a
RIGHT JOIN tbl_dept b ON a.dept_id = b.id
3.3 Full external connection
SELECT
*
FROM
tbl_emp a
LEFT JOIN tbl_dept b ON a.dept_id = b.id
WHERE
b.id IS NULL
UNION
SELECT
*
FROM
tbl_emp a
RIGHT JOIN tbl_dept b ON a.dept_id = b.id
WHERE
a.dept_id IS NULL