Table of Contents
1. Cartesian product
2. Classification
(1) Classification by age
(2) Classification by function
3. Equivalent join
Home Database Mysql Tutorial Analysis of connection query examples in MySQL

Analysis of connection query examples in MySQL

May 27, 2023 pm 02:34 PM
mysql

    1. Cartesian product

    Table 1 has m rows of data, Table 2 has n rows of data, and the query result has m*n rows of data.

    2. Classification

    (1) Classification by age

    sql92 standard: only supports inner joins

    sql99 standard (recommended): supports inner joins, Outer joins (left outer joins and right outer joins), cross joins

    (2) Classification by function

    Inner joins: equivalent joins, non-equivalent joins, self-joins

    Outer join: left outer join, right outer join, full outer join

    Cross join

    3. Equivalent join

    (1) Query the girl’s name and its corresponding Boyfriend’s name

    SELECT 
        girlname,
        boyname
    FROM
        boys,
        girls
    WHERE
        girls.boyfriend_id=boys.girlfriend_id;
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    (2) Query the employee name (last_name) and the corresponding department name (department_name)------------- (query based on the associated id)

    SELECT 
        last_name,
        department_name
    FROM
        JDSC,
        SNSC
    WHERE
        JDSC.`id`=SNSC.`id`;
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    (3) Query employee name (last_name), job type number (job_id), job type name (job_title) (employee table: JDSC work table: JOBSC) ------ Alias ​​the table to improve simplicity , to avoid ambiguity

    #"e.job_id"是为了避免歧义
    SELECT
        last_name,
        e.job_id,
        job_title
    FROM
        ESC e,
        JOBSC j
    WHERE
        e.`job_id`=j.`job_id`;
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    Note:

    If an alias is given to the table, the queried fields cannot be qualified by the original table name.

    The order of the above two tables can be interchanged

    (4) Query the names of employees and departments with bonuses

    #员工名:last_name
    #部门名:department_name
    #奖金率:commissiom_pct
    SELECT
        last_name,
        department_name,
        commissiom_pct
    FROM
        employees e,
        department d
    WHERE
        e.`department_id`=d.`department_id`
    AND
        e.`commissiom_pct` IS NOT NULL;
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    (5) Group query-mdash;Query Number of departments in each city

    #城市分组表名:city
    #部门分组表名:departments 
    SELECT
        COUNT(*) 个数,
        city
    FROM
        departments d,
        locations l
    WHERE
        d.`location_id`=l.`location_id`
    GROUP BY
        city;
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    (6) Sorting - Query the job name and number of employees for each type of work, and sort them in descending order by the number of employees

    SELECT 
        job_title,
        COUNT(*)
    FROM
        emloyees e,
        jobs j
    WHERE
        e.`job_id`=j.`job_id`
    GROUP BY
        job_title
    ORDER BY
        COUNT(*) DESC;
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    (7) Three Table connection-query employee name, department name and city

    SELECT
        last_name,
        department_name,
        city
    FROM
        employees e,
        departments d,
        locations l
    WHERE
        e.`department_id`=d.`department_id`
    AND
        d.`location_id`=l.`location_id`
    AND 
        city LIKE 's%';
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