How to make subquery in mysql?

In mysql, a subquery refers to nesting a query statement within another query statement. It can be implemented with the WHERE clause in the SELECT, UPDATE and DELETE statements. The syntax format of the WHERE clause is: "WHERE (another query statement)".

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Subquery is a comparison in MySQL Commonly used query methods, multi-table queries can be implemented through subqueries. Subquery refers to nesting one query statement within another query statement. Subqueries can be used in SELECT, UPDATE, and DELETE statements and can be nested at multiple levels. In actual development, subqueries often appear in the WHERE clause.

The syntax format of subquery in WHERE is as follows:

WHERE <表达式> <操作符> (子查询语句)

Among them, the operator can be a comparison operator and keywords such as IN, NOT IN, EXISTS, NOT EXISTS.

1) IN | NOT IN

When the expression is equal to a value in the result set returned by the subquery, TRUE is returned, otherwise FALSE is returned; if used Keyword NOT, the return value is exactly the opposite.

2) EXISTS | NOT EXISTS

is used to determine whether the result set of the subquery is empty. If the result set of the subquery is not empty, TRUE is returned, otherwise it is returned FALSE; if keyword NOT is used, the returned value is exactly the opposite.

Example 1

Use subquery to query the names of students taking Java courses in the tb_students_info table and tb_course table. The SQL statement and running results are as follows.

mysql> SELECT name FROM tb_students_info 
    -> WHERE course_id IN (SELECT id FROM tb_course WHERE course_name = 'Java');
+-------+
| name  |
+-------+
| Dany  |
| Henry |
+-------+
2 rows in set (0.01 sec)

The results show that only Dany and Henry are taking Java courses. The above query process can also be divided into the following two steps to achieve the same effect.

1) First execute the inner query separately to find out the ID of the course Java in the tb_course table. The SQL statement and running results are as follows.

mysql> SELECT id FROM tb_course 
    -> WHERE course_name = 'Java';
+----+
| id |
+----+
|  1 |
+----+
1 row in set (0.00 sec)

You can see that the value of the id field that meets the conditions is 1.

2) Then execute the outer query to query the names of students whose course_id is equal to 1 in the tb_students_info table. The SQL statements and running results are as follows.

mysql> SELECT name FROM tb_students_info 
    -> WHERE course_id IN (1);
+-------+
| name  |
+-------+
| Dany  |
| Henry |
+-------+
2 rows in set (0.00 sec)

Conventionally, the outer SELECT query is called the parent query, and the query embedded in parentheses is called a subquery (subqueries must be placed within parentheses). When MySQL processes the SELECT statement in the above example, the execution flow is: first execute the subquery, and then execute the parent query.

Example 2

Similar to Example 1, use the NOT IN keyword in the SELECT statement to query the names of students who have not taken Java courses. The SQL statement and running results are as follows .

mysql> SELECT name FROM tb_students_info 
    -> WHERE course_id NOT IN (SELECT id FROM tb_course WHERE course_name = 'Java');
+--------+
| name   |
+--------+
| Green  |
| Jane   |
| Jim    |
| John   |
| Lily   |
| Susan  |
| Thomas |
| Tom    |
| LiMing |
+--------+
9 rows in set (0.01 sec)

It can be seen that the running result is exactly the opposite of Example 1. The students who have not studied Java courses are students other than Dany and Henry.

Example 3

Use the = operator to query the names of all students studying Python courses in the tb_course table and tb_students_info table. The SQL statement and running results are as follows.

mysql> SELECT name FROM tb_students_info
    -> WHERE course_id = (SELECT id FROM tb_course WHERE course_name = 'Python');
+------+
| name |
+------+
| Jane |
+------+
1 row in set (0.00 sec)

The results show that the only student studying the Python course is Jane.

Example 4

Use the <> operator to query the names, SQL statements and running results of students who have not taken Python courses in the tb_course table and tb_students_info table as follows.

mysql> SELECT name FROM tb_students_info
    -> WHERE course_id <> (SELECT id FROM tb_course WHERE course_name = &#39;Python&#39;);
+--------+
| name   |
+--------+
| Dany   |
| Green  |
| Henry  |
| Jim    |
| John   |
| Lily   |
| Susan  |
| Thomas |
| Tom    |
| LiMing |
+--------+
10 rows in set (0.00 sec)

It can be seen that the running result is just the opposite of Example 3. The students who have not studied Python courses are students other than Jane.

Example 5

Query whether there is a course with id=1 in the tb_course table. If it exists, query the records in the tb_students_info table. The SQL statement and running results are as follows .

mysql> SELECT * FROM tb_students_info
    -> WHERE EXISTS(SELECT course_name FROM tb_course WHERE id=1);
+----+--------+------+------+--------+-----------+
| id | name   | age  | sex  | height | course_id |
+----+--------+------+------+--------+-----------+
|  1 | Dany   |   25 | 男   |    160 |         1 |
|  2 | Green  |   23 | 男   |    158 |         2 |
|  3 | Henry  |   23 | 女   |    185 |         1 |
|  4 | Jane   |   22 | 男   |    162 |         3 |
|  5 | Jim    |   24 | 女   |    175 |         2 |
|  6 | John   |   21 | 女   |    172 |         4 |
|  7 | Lily   |   22 | 男   |    165 |         4 |
|  8 | Susan  |   23 | 男   |    170 |         5 |
|  9 | Thomas |   22 | 女   |    178 |         5 |
| 10 | Tom    |   23 | 女   |    165 |         5 |
| 11 | LiMing |   22 | 男   |    180 |         7 |
+----+--------+------+------+--------+-----------+
11 rows in set (0.01 sec)

As can be seen from the results, there is a record with id=1 in the tb_course table, so the EXISTS expression returns TRUE. After receiving TRUE, the outer query statement queries the table tb_students_info and returns all records.

The EXISTS keyword can be used together with other query conditions. The conditional expression and the EXISTS keyword are connected with AND and OR.

Example 6

Query whether there is a course with id=1 in the tb_course table. If it exists, query the records with the age field greater than 24 in the tb_students_info table. SQL statement And the running results are as follows.

mysql> SELECT * FROM tb_students_info
    -> WHERE age>24 AND EXISTS(SELECT course_name FROM tb_course WHERE id=1);
+----+------+------+------+--------+-----------+
| id | name | age  | sex  | height | course_id |
+----+------+------+------+--------+-----------+
|  1 | Dany |   25 | 男   |    160 |         1 |
+----+------+------+------+--------+-----------+
1 row in set (0.01 sec)

The results show that a record was queried from the tb_students_info table, and the age field value of this record is 25. The inner query statement retrieves records from the tb_course table and returns TRUE. The outer query statement starts the query. According to the query conditions, query the records with age greater than 24 from the tb_students_info table.

Expand

The functionality of subqueries can also be accomplished through table joins, but subqueries make SQL statements easier to read and write.

Generally speaking, table joins (inner joins and outer joins, etc.) can be replaced by subqueries, but the reverse is not necessarily true. Some subqueries cannot be replaced by table joins. Subqueries are more flexible, convenient, and diverse in form, and are suitable as filter conditions for queries, while table joins are more suitable for viewing data in connected tables.

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