概述
今天主要对PG数据库的一些基本SQL语句用法做个介绍，做个简单了解，也做备忘！
下面主要用例子来说明。
1、建表语句
create table test (
id int8 primary key,
info text,
crt_time timestamp
);
注意保留字
2、select into & create table as
postgres=# select * into table new_tbl from pg_class;
postgres=# create table tbl_1 as select * from pg_class;
3、插入\更新\删除\查询
insert into tbl (xx,xx) values (xx,xx);
update tbl set xx=xx where xxx;
delete from tbl where xxx=xxx;
select xx from xx where xx...;
如果是delete|update limit，则：
update tbl set xx=xx where ctid = any ( array (select ctid from tbl where xx limit ? for update));
delete from tbl where ctid = any ( array (select ctid from tbl where xx limit ? for update));
4、批量DML
insert into xx values (),(),...();
copy xx from stdin;
copy xx from ‘file‘;
pg_bulkload
update t set info=t1.info,crt_time=t1.crt_time from t1,t2 where (t.id=t1.id) and t1.id=t2.id;
update tbl_1 set relname=tmp.rel from (values (1,‘test1‘),(2,‘test2‘)) tmp (id, rel) where tmp.id=tbl_1.id;
delete from t using t1 where t.id=t1.id;
delete from tbl_1 using (values (1),(2)) tmp (rel) where tmp.rel=tbl_1.reltype;
注意update , delete 批量操作，JOIN不是一一对应时，更新目标可能会随机匹配。
5、DB端copy+客户端copy
• https://github.com/digoal/blog/blob/master/201805/20180516_03.md
• https://github.com/digoal/blog/blob/master/201805/20180510_01.md
5.1、copy为什么快？
协议：
5.2、DB 端copy
copy tbl to ‘file‘;
copy (SQL) to ‘file‘;
copy tbl from ‘file‘;
5.3、客户端copy
copy tbl from stdin;c
opy (SQL) to stdout;
copy tbl to stdout;
psql (\copy to | from); -- copy协议
6、排序+ offset limit
select * from tbl_1 order by relname nulls first;
select * from tbl_1 order by relname nulls last;
select * from tbl_1 order by relname;
select * from tbl_1 order by relname limit 10 offset 10;
select * from tbl_1 order by relname::text collate "C";
7、聚合+解耦合
select string_agg(relname,‘,‘ order by xx) from tbl_1;
select g,avg(c1) from tbl group by g;
8、distinct
select distinct relname,relnamespace from pg_class;SELECT id, COUNT_DISTINCT(val) FROM test_table GROUP BY 1;
select count(distinct (relname,relnamespace)) from pg_class;select distinct on (c3) c2,c3 from tbl;
9、INNER|OUTER JOIN
•inner
select * from t1 join t2 on (t1.x=t2.x) where xxxx;
• left
1)scan filter
select t1.*,t2.* from t1 left join t2 on (t1.x=t2.x) where t1.x=x;
2)join filter
select t1.*,t2.* from t1 left join t2 on (t1.x=t2.x and t1.x=x);
• right
把上面的left join改成right join即可，这里就不多说了。