这样我的两个字段就都成为主键了。
创建符合主键的同时建立外键：
myd=# create table mapping(adminId integer references admin(id),roleId integer references role(id),roleId));
这样我的的两个字段既是主键的组成部分，又分别参照了别的表的字段。
创建符合主键的同时建立外键并级联删除级联跟新：
myd=# create table mapping(adminid integer references admin(id) on delete cascade on update cascade,roleid integer references role(id) on delete cascade on update cascade,primary key(adminid,roleid));
下面是提示信息：
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "mapping_pkey" for table "mapping"
CREATE TABLE
多表连接：
myd=# select (admin.id,admin.name,role.name) from admin,mapping,role where admin.id=mapping.adminid and mapping.roleid=role.id;
多表连接同时指定列的别名：
myd=# select admin.id as id,admin.name as name,role.name as role from admin,role where admin.id=mapping.adminid and mapping.roleid=role.id;
对已经存在的表添加主键：
alter table t add primary key(id);
其中t为表明，id为要设置为主键的字段，注意，如果字段中有重复值，会创建失败。
distinct去重
select count(distinct col) from A;
select count(1) from (select 1 from A group by col) alias;
copy导入文件和导出文件：
copY 命令可以快速的导入数据到 Postgresql 中，文件格式类似CVS之类。适合批量导入数据，比 \i 和恢复数据表快。
导出表数据到文件或 STDOUT ：
copY tablename [(column [,...])]
TO {'filename' | STDOUT}
[[WITH]
[BINARY]
[OIDS]
[DELIMITER [AS] 'delimiter']
[NULL [AS] 'null string']
[CSV [HEADER]
[QUOTE [AS] 'quote']
[ESCAPE [AS] 'escape']
[FORCE NOT NULL column [,...]]
导入文件或者 STDIN 到表中：
copY tablename [(column [,...])]
FROM {'filename' | STDIN}
[[WITH]
[BINARY]
[OIDS]
[DELIMITER [AS] 'delimiter']
[NULL [AS] 'null string']
[CSV [HEADER]
[QUOTE [AS] 'quote']
[ESCAPE [AS] 'escape']
[FORCE QUOTE column [,...]]
导出表 employee 到默认输出 STDOUT：
psql> copY employee TO STDOUT;
1 JG100011 Jason Gilmore [email protected]
2 RT435234 Robert Treat [email protected]
3 GS998909 Greg Sabino Mullane [email protected]
4 MW777983 Matt Wade [email protected]
导出表 employee 到 sql 文件：
psql> copY employee TO '/home/smallfish/employee.sql';
从文件导入数据：
psql> copY employeenew FROM '/home/smallfish/employee.sql';
psql> SELECT * FROM employeenew;
employeeid | employeecode | name | email
------------+--------------+---------------------+---------------
1 | JG100011 | Jason Gilmore | [email protected]
2 | RT435234 | Robert Treat | [email protected]
3 | GS998909 | Greg Sabino Mullane | [email protected]
4 | MW777983 | Matt Wade | [email protected]
(4 rows)
输出对象ID（OIDS）：
psql> copY employee TO STDOUT OIDS;
24627 1 GM100011 Jason Gilmore [email protected]
24628 2 RT435234 Robert Treat [email protected]
24629 3 GS998909 Greg Sabino Mullane [email protected]
24630 4 MW777983 Matt Wade [email protected]
指定导出间隔符，默认是 \t ，这里为 | ：
psql>copY employee TO STDOUT DELIMITER '|';
1|GM100011|Jason Gilmore|[email protected]
2|RT435234|Robert Treat|[email protected]
3|GS998909|Greg Sabino Mullane|[email protected]
4|MW777983|Matt Wade|[email protected]
导入文件数据，指定间隔符为 | ：
psql> copY employeenew FROM '/home/smallfish/employee.sql' DELIMITER |;
导出指定字段的数据：
psql> copY employee (name,email) TO STDOUT;
Jason Gilmore [email protected]
Robert Treat [email protected]
Greg Sabino Mullane [email protected]
Matt Wade [email protected]
为 NULL 字段设置默认值：
psql> copY employee TO STDOUT NULL 'no email';
Jason Gilmore no email
Robert Treat [email protected]
Greg Sabino Mullane [email protected]
Matt Wade no email
导出为CVS格式：
psql> copY employee (name,email) TO '/home/smallfish/employee.csv' CSV HEADER;
MysqL导出数据到文件： select *(或者字段名) from tablename into outfile "/root/aa.txt";
MysqL导入到MysqL中： load data infile "/root/aa.txt" into table tablename; table需要提前创建好。
MySQL INTO OUTFILE/INFILE导出导入数据
--使用SELECT ... INTO OUTFILE 以逗号分隔字段的方式将数据导入到一个文件中:
SELECT * INTO OUTFILE 'D:\\log1.txt' FIELDS TERMINATED BY ',' FROM log.log1
--将刚刚导出的文件log1.txt导入到表log1相同结构的log2中:
LOAD DATA INFILE 'D:\\log1.txt' INTO TABLE aa.log2 FIELDS TERMINATED BY ','
-- 使用select * into outfile 导出:
SELECT * INTO OUTFILE 'd:\\test.txt' FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' LInes TERMINATED BY '\n' FROM test.table
-- 导入
LOAD DATA INFILE '/tmp/fi.txt' INTO TABLE test.fii FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' LInes TERMINATED BY '\n'
FIELDS TERMINATED BY','字段间分割符 OPTIONALLY ENCLOSED BY'"'将字段包围 对数值型无效 LInes TERMINATED BY'\n'换行符
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