mariadb的select語句

mariadb的查詢流程圖

select語句的從句分析順序：from(過濾表)-->where(過濾行)-->group by(分組)-->having(分組過濾)-->order by(排序)--

>select(選取欄位)-->limit(查詢限制)-->最終結果

DISTINCT: 數據去重

SQL_CACHE: 顯式指定存儲查詢結果於緩存之中

SQL_NO_CACHE: 顯式查詢結果不予緩存

show global variables like '%query%';

query_cache_type             | ON    表示緩存開啟

query_cache_size             | 0     表示緩存空間大小，如果為0則不緩存

query_cache_type的值為'DEMAND'時，顯式指定SQL_CACHE的SELECT語句才會緩存；其它均不予緩存

緩存並不會緩存所有查詢結果，例如select now()；就不會緩存

WHERE子句：指明過濾條件以實現“選擇”的功能

算術操作符：+, -, *, /, %

比較操作符：=, !=, <>, <=>, >, >=, <, <=

BETWEEN min_num AND max_num

IN (element1, element2, ...)

IS NULL

IS NOT NULL

LIKE:

　　%: 任意長度的任意字元

　　_：任意單個字元

RLIKE

邏輯操作符：　　NOT,AND,OR

GROUP：根據指定的條件把查詢結果進行“分組”以用於做“聚合”運算：avg(), max(), min(), count(), sum()

HAVING: 對分組聚合運算後的結果指定過濾條件

ORDER BY: 根據指定的欄位對查詢結果進行排序：升序ASC    降序：DESC

LIMIT [[offset,]row_count]：對查詢的結果進行輸出行數數量限制

例如：

select name,age from students where age/2=11;

select name,age from students where age+30>50;

select distinct gender from students;

select name as stuname from students;

select name,classid from students where classid is null;

select avg(age),gender from students group by gender;

select avg(age) as ages,gender from students group by gender having ages>20;

select count(stuid) as NO,classid from students group by classid;

select count(stuid) as NO,classid from students group by classid having NO>2;

select name,age from students order by age limit 10,10;  (第一個表示偏移10個，第二個表示取10個)

多表查詢：

　　交叉連接：笛卡爾乘積(最消耗資源的一種查詢)　　例如：select * from students,teachers; 如果students有20行，teachers也有20行，則顯示400行

　　內連接：

　　　　等值連接：讓表之間的欄位以“等值”建立連接關係；

　　　　不等值連接

　　　　自然連接

　　　　自連接

　　外連接：

　　　　左外連接　　例如：FROM tb1 LEFT JOIN tb2 ON tb1.col=tb2.col

　　　　右外連接　　例如：FROM tb1 RIGHT JOIN tb2 ON tb1.col=tb2.col

等值連接：

select s.name as studentname,t.name as teachername from students as s,teachers as t where s.teacherid=t.tid;(這個夠複雜)

select s.name,c.class from students as s,classes as c where s.classid=c.classid;

左外連接：

select s.name,c.class from students as s left join classes as c on s.classid=c.classid;

以左表作為基準

select s.name,c.class from students as s right join classes as c on s.classid=c.classid;

以右表為基準

子查詢：在查詢語句嵌套著查詢語句,基於某語句的查詢結果再次進行的查詢 (mariadb對子查詢優化不夠，建議一般情況不使用)

select name,age from students where age>(select avg(age) from students);

select name,age from students where age in (select age from teachers);

select s.ages,s.classid from (select avg(age) as ages,classid from students where classid is not null group by classid) as s where s.ages>30;

聯合查詢：UNION

select name,age from students union select name,age from teachers;