SQL語句練習

小美是一所中學的資訊科技老師，她有一張 seat 座位表，平時用來儲存學生名字和與他們相對應的座位 id。

其中縱列的 id 是連續遞增的

小美想改變相鄰倆學生的座位。

±--------±--------+

| id | student |

±--------±--------+

| 1 | abbot |

| 2 | doris |

| 3 | emerson |

| 4 | green |

| 5 | jeames |

±--------±--------+

假如資料輸入的是上表，則輸出結果如下：

±--------±--------+

| id | student |

±--------±--------+

| 1 | doris |

| 2 | abbot |

| 3 | green |

| 4 | emerson |

| 5 | jeames |

±--------±--------+

select *

from (select id - 1 as id, student
from seat
where id%2 = 0
union
select id + 1 as id, student
from seat
where id%2 = 1
and (id + 1) <= (select count(*) from seat)
union
select id as id, student
from seat
where id%2 = 1
and (id + 1) > (select count(*) from seat)) as t1
order by id asc;

select (case

when mod(id, 2) = 1 and id = (select count(*) from seat) then
idwhen mod(id, 2) = 1 then
id + 1
else
id - 1
end) as id,
student
from seat
order by id;

找出每個部門工資前三高的員工。

select d.name as department,e.name as employee,e.salary as salary

from employee as e
inner join department as d
on e.departmentid = d.id
where (
select count(distinct salary)
from employee
where salary > e.salary 
and departmentid = e.departmentid
) < 3
order by e.departmentid,salary desc;

擴充套件：sql的left join 、right join 、inner join之間的區別

left join(左聯接)返回包括左表中的所有記錄和右表中聯結字段相等的記錄

right join(右聯接)返回包括右表中的所有記錄和左表中聯結字段相等的記錄

inner join(等值連線)只返回兩個表中聯結字段相等的行

交換性別

update salary

set*** = case ***
when 'm' then 'f'
else 'm'
end;

shell指令碼

4.統計詞頻

awk ') \|[0-9]\-\)[0-9]\-[0-9]\$' file.txt

awk 'nr==10' file2.txt

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