当我们需要的数据需从两张及以上的表中查询出来时，这时就需要连接查询。

    语法：

    1. SELECT
    2.     <字段> 
    3. FROM
    4.     <表名> 
    5. WHERE
    6.     <表名 1 >.<字段> = <表名 2 >.字段;


    例子：

    1. # 查询员工和对应的部门名。
    3. SELECT
    4.     last_name,
    5.     department_name 
    6. FROM
    7.     employees,
    8.     departments 
    9. WHERE
    10.     employees.department_id = departments.department_id;
    12. # 查询员工名、工种号、工种名。
    14. SELECT
    15.     last_name,
    16.     employees.job_id,
    17.     job_title 
    18. FROM
    19.     employees,
    20.     jobs 
    21. WHERE
    22.     employees.job_id = jobs.job_id;
    25. # 查询有奖金的员工名、部门名。
    27. SELECT
    28.     last_name,
    29.     department_name 
    30. FROM
    31.     employees,
    32.     departments 
    33. WHERE
    34.     departments.department_id = employees.department_id 
    35.     AND employees.commission_pct IS NOT NULL;
    38. # 查询城市名中第二个字符为 o 的部门名和城市名。
    40. SELECT
    41.     department_name,
    42.     city 
    43. FROM
    44.     departments,
    45.     locations 
    46. WHERE
    47.     departments.location_id = locations.location_id 
    48.     AND city LIKE '_o%';
    50. # 查询每个城市的部门个数。
    52. SELECT
    53.     COUNT(*) AS 数量,
    54.     city 
    55. FROM
    56.     locations,
    57.     departments 
    58. WHERE
    59.     locations.location_id = departments.location_id 
    60. GROUP BY
    61.     city;
    63. # 查询有奖金的每个部门的部门名和部门的领导编号和该部门的最低工资。
    65. SELECT
    66.     department_name,
    67.     departments.manager_id,
    68.     MIN( salary ) 
    69. FROM
    70.     departments,
    71.     employees 
    72. WHERE
    73.     departments.department_id = employees.department_id 
    74.     AND employees.commission_pct IS NOT NULL 
    75. GROUP BY
    76.     department_name,
    77.     departments.manager_id;
    80. # 查询每个工种的工种名和员工的个数，并且按员工的个数降序。
    82. SELECT
    83.     job_title,
    84.     COUNT(*) 
    85. FROM
    86.     employees,
    87.     jobs 
    88. WHERE
    89.     employees.job_id = jobs.job_id 
    90. GROUP BY
    91.     job_title  
    92. ORDER BY
    93.     COUNT(*) DESC;
    95. # 查询员工名、部门名和所在的城市。
    97. SELECT
    98.     last_name,
    99.     department_name,
    100.     city 
    101. FROM
    102.     employees,
    103.     departments,
    104.     locations 
    105. WHERE
    106.     employees.department_id = departments.department_id 
    107.     AND departments.location_id = locations.location_id;
    110. # 查询员工的工资和工资级别（非等值连接）。
    112. SELECT
    113.     salary,
    114.     grade_level 
    115. FROM
    116.     employees,
    117.     job_grades 
    118. WHERE
    119.     salary BETWEEN job_grades.lowest_sal 
    120.     AND job_grades.highest_sal;


    自连接，连接的两张表都是同一张表。是一种特殊的等值连接。

    例子：

    1. # 自连接
    2. # 查询员工名和上级的名称
    4. SELECT
    5.     e.employee_id,
    6.     e.last_name,
    7.     m.employee_id,
    8.     m.last_name 
    9. FROM
    10.     employees AS e,
    11.     employees AS m 
    12. WHERE
    13.     e.manager_id = m.manager_id;


    参考：