CREATE TABLE users (
  id INT,
  name VARCHAR(50)
);

PostgreSQL
CREATE TABLE users (
  id INTEGER,
  name VARCHAR(50)
);

字符串连接
MySql:concat(str1,str2,...)
PostgreSQL:||

mysql
SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM users;

PG
SELECT first_name || ' ' || last_name AS full_name FROM users;


自增列
mysql  AUTO_INCREMENT   
PG   SERIAL

LIMIT子句

SELECT * FROM users LIMIT 10;

SELECT * FROM users LIMIT 10 OFFSET 0;



子查询
SELECT * FROM users WHERE id IN (SELECT user_id FROM orders);

SELECT * FROM users WHERE id = ANY (SELECT user_id FROM orders);

获取当前时间 
select now()


获取时间类型的年月日

MySql:
YEAR(NOW())   MONTH(NOW()) DAY(NOW()) 
HOUR(NOW())  MINUTE(NOW())  SECOND(NOW())


PostgreSQL:
to_char(now(),'yyyy')
yyyy-MM
yyyy-MM-dd


判断字符是否为空（如果value1不为空，结果返回value1；如果value1为空，结果返回value2）
SqlServer:isnull(value1,value2)
MySql:isnull(value1,value2)
PostgreSQL:COALESCE(value1,value2)


取前几条数据  num（条数）
SqlServer:top  
select top num * from table
MySql:top 
select top num * from table
PostgreSQL:limit    
select * from table limit num

列名与关键字冲突
SqlServer:  []
MySql:   ``
PostgreSQL: ""


生成GUID
SqlServer:newid()
MySql:uuid()
PostgreSQL:gen_random_uuid()   需要安装uuid-ossp 扩展pgcrypto 扩展

获取字符串长度
SqlServer:len()
MySql:char_length()
PostgreSQL:char_length()