1.DQL單表數(shù)據(jù)查詢
--------------------------------------------------------------------------------------------old guo
1.SELECT
1.SELECT 單獨(dú)使用
- 查詢數(shù)據(jù)庫的參數(shù)
SELECT @@port;
SELECT @@datadir;
SELECT @@basedir;
SELECT @@innodb_flush_log_at_trx_commit;
SHOW VARIABLES LIKE '%trx%';
- 調(diào)用內(nèi)置函數(shù).
SELECT DATABASE(); 查詢當(dāng)前所在數(shù)據(jù)庫
SELECT NOW(); 查看時(shí)間
SELECT CONCAT(USER,"@",HOST) FROM mysql.user; 合并
SELECT GROUP_CONCAT(xid) FROM student; 將列合并
SELECT SUM(xid) FROM student; xid列求和
- 簡易計(jì)算器
SELECT 4*5;
2.select 配合其他子句使用
子句列表介紹
FROM -- 查詢對(duì)象(表,視圖)
WHERE -- 過濾子句(grep)
GROUP BY -- 分組子句(統(tǒng)計(jì)分析類)
ORDER BY -- 排序子句
HAVING -- 后過濾子句
LIMIT -- 限制子句(分頁子句)
1.配合FROM應(yīng)用
- 1.查詢表中所有數(shù)據(jù)(cat)
SELECT * FROM city; - 查詢name和population信息 (awk取列)
SELECT NAME,population FROM city;
- 查詢name和population信息 (awk取列)
2.select+ from + where(grep)使用
- where 配合等值查詢
1.查詢中國所有的城市信息
SELECT * FROM city WHERE countrycode='CHN';
2.查詢ID為100的城市信息
SELECT * FROM city WHERE id=100;
3.查詢 中國河北省的城市信息
SELECT * FROM city WHERE countrycode='CHN' AND district='hebei';
4.查詢 中國或者美國的城市
SELECT * FROM city WHERE countrycode='CHN' OR countrycode='USA';
或者:
SELECT * FROM city WHERE countrycode IN ('CHN','USA');
或者:
SELECT * FROM city WHERE countrycode='CHN' UNION ALL SELECT * FROM city WHERE countrycode='USA' ; - where 配合范圍查詢
1.查詢?nèi)丝跀?shù)量小于100人的城市
SELECT * FROM city WHERE population<100;
2.查詢?nèi)丝跀?shù)量100w-200w之間的
SELECT * FROM city WHERE population>=1000000 AND population<=2000000 ;
或者:
SELECT * FROM city WHERE population BETWEEN 1000000 AND 2000000 ;
3.查詢國家代號(hào)是CH開頭的城市信息
SELECT * FROM city WHERE countrycode LIKE 'CH%';
3.group by 分組子句+聚合函數(shù)應(yīng)用
聚合函數(shù)?
COUNT() -- 計(jì)數(shù)
SUM() -- 求和
AVG() -- 求平均值
MAX() -- 求最大值
MIN() -- 最小值
GROUP_CONCAT() -- 聚合列值
- 結(jié)果集顯示特點(diǎn): 必須是1v1,不能是1vN
1.統(tǒng)計(jì)一下每個(gè)國家的人口總數(shù)
SELECT countrycode,SUM(population) FROM city GROUP BY countrycode;
2.統(tǒng)計(jì)中國每個(gè)省的人口總數(shù)
SELECT district,SUM(population) FROM city WHERE countrycode='CHN' GROUP BY district;
3.統(tǒng)計(jì)下中國每個(gè)省的城市個(gè)數(shù)及城市名.
SELECT district,COUNT(NAME),GROUP_CONCAT(NAME) FROM city WHERE countrycode='CHN' GROUP BY district;
4.統(tǒng)計(jì)每個(gè)國家城市個(gè)數(shù)
SELECT countrycode ,COUNT(NAME) FROM city GROUP BY countrycode;
4.having 后判斷
1.統(tǒng)計(jì)中國每個(gè)省的人口總數(shù),只顯示總?cè)丝跀?shù)大于500w的省信息.
SELECT district,SUM(population) FROM city WHERE countrycode='CHN' GROUP BY district HAVING SUM(population) >=5000000;
6.order by 排序子句
1.查詢中國所有城市信息,人口數(shù)從大到小排序輸出.
SELECT * FROM city WHERE countrycode='CHN' ORDER BY population DESC ;
2..查詢中國所有城市信息,按城市名排序.
SELECT * FROM city WHERE countrycode='CHN' ORDER BY NAME;
3.查詢中國所有省的總?cè)丝?并按總?cè)丝跀?shù)從大到小排序輸出.
SELECT district,SUM(population) FROM city WHERE countrycode='CHN' GROUP BY district ORDER BY SUM(population) DESC;
5.limit 分頁限制子句
limit m offset n 跳過n行顯示m行
查詢中國所有省的總?cè)丝?并按總?cè)丝跀?shù)從大到小排序輸出.
SELECT district,SUM(population) FROM city WHERE countrycode='CHN' GROUP BY district ORDER BY SUM(population) DESC LIMIT 5 OFFSET 1;
SELECT district,SUM(population) FROM city WHERE countrycode='CHN' GROUP BY district ORDER BY SUM(population) DESC LIMIT 10;
SELECT district,SUM(population) FROM city WHERE countrycode='CHN' GROUP BY district ORDER BY SUM(population) DESC LIMIT 1,5;
注意: LIMIT 謹(jǐn)慎使用, 500w+的表.
LIMIT 5000000,100
一般會(huì)改為明確查找范圍
2.DQL多表數(shù)據(jù)查詢
1.介紹
內(nèi)連接 inner join
外連接 left join right join
笛卡爾
2.作用
聚合多張表數(shù)據(jù),實(shí)現(xiàn)查詢需求
3.多表連接的語法
- 內(nèi)連接(交集)
FROM A
INNER JOIN B
ON A.x=B.y
- 外連接
FROM A
LEFT JOIN B
ON A.x=B.y
FROM A
RIGHT JOIN B
ON A.x=B.y
1.3.3 笛卡爾乘積
FROM A
JOIN B
4.多表連接例子
套路1. 找關(guān)聯(lián)表 2. 找關(guān)系列
- 查詢?nèi)丝跀?shù)小于100人城市名,國家名,國土面積?
SELECT
city.name,
country.name,
country.surfacearea,
city.population
FROM
city
JOIN country
ON city.countrycode = country.code
WHERE city.population < 100
- 統(tǒng)計(jì)查詢每位學(xué)員的平均分
SELECT student.xid ,student.xname,AVG(score.score)
FROM student
JOIN score
ON student.xid=score.xid
GROUP BY student.xid,student.xname
- 統(tǒng)計(jì)每位學(xué)員學(xué)習(xí)了幾門課
SELECT student.xid ,student.xname,COUNT(score.score)
FROM student
JOIN score
ON student.xid=score.xid
GROUP BY student.xid,student.xname
- 每位老師教的學(xué)生的個(gè)數(shù)和姓名列表
SELECT teacher.tname,COUNT(student.xid),GROUP_CONCAT(student.xname)
FROM student
JOIN score
ON student.xid=score.xid
JOIN course
ON score.cid=course.cid
JOIN teacher
ON course.tid=teacher.tid
GROUP BY teacher.tid,teacher.tname;
5.left/right join 外連接應(yīng)用
- 一般應(yīng)用在強(qiáng)制驅(qū)動(dòng)表時(shí),強(qiáng)制小結(jié)果集驅(qū)動(dòng)大表
- 生產(chǎn)中可以使用left join 強(qiáng)制 驅(qū)動(dòng)表.盡量減少next loop的出現(xiàn).
- 為什么要強(qiáng)制? inner join 優(yōu)化器 自動(dòng)選擇, 按照索引選擇的幾率較大
6 補(bǔ)充 別名的應(yīng)用 .
- 列別名
SELECT
course.`cname` AS 課程名稱,
GROUP_CONCAT(CASE WHEN score.`score` >= 85 THEN student.xname END ) AS "優(yōu)秀",
GROUP_CONCAT(CASE WHEN score.`score` >=70 AND score.`score` < 85 THEN student.xname END) AS "良好",
GROUP_CONCAT(CASE WHEN score.`score` >=60 AND score.`score` <70 THEN student.xname END )AS "一般",
GROUP_CONCAT(CASE WHEN score.`score` <60 THEN student.xname END ) AS "不及格"
FROM student
JOIN score
ON student.xid = score.xid
JOIN course
ON score.`cid`=course.`cid`
GROUP BY course.`cid`;
說明: 1. 為了顯示的好看. 2. 可以在 having 或 order by 子句中調(diào)用
- 表別名
SELECT
CONCAT(te.tname,"_",te.tid) AS "教師名",GROUP_CONCAT(CONCAT(st.xname,":",sc.score))
FROM teacher as te
JOIN course as co
ON te.`tid`=co.`tid`
JOIN score as sc
ON co.`cid`=sc.`cid`
JOIN student as st
ON sc.`xid`=st.`xid`
WHERE sc.`score`<60
GROUP BY te.tid,te.tname;
