MySQL ClickHouse不同于SQL的语法介绍

时间:2022-11-03 小基基o_O 人气:1

下面只展示和MySQL或HIVE区别较大的语法

例如ClickHouse建库语法和MySQL几乎1卵样的不予展示

ClickHouse的JOIN逻辑

# 创建左表
CREATE TABLE default.temp_left(
    `a` String COMMENT '匹配键'
)ENGINE = MergeTree
ORDER BY (a);
INSERT INTO default.temp_left
SELECT 'A1' AS a UNION ALL
SELECT 'A1' AS a UNION ALL
SELECT 'A2' AS a UNION ALL
SELECT 'A3' AS a;
# 创建右表
CREATE TABLE default.temp_right(
    `a` String COMMENT '匹配键',
    `b` Nullable(UInt32),
    `c` UInt32
)ENGINE = MergeTree
ORDER BY (a);
INSERT INTO default.temp_right
SELECT 'A2' AS a,9 AS b,8 AS c UNION ALL
SELECT 'A3' AS a,9 AS b,8 AS c UNION ALL
SELECT 'A4' AS a,9 AS b,8 AS c;
# 左联
SELECT *
FROM default.temp_left le
LEFT JOIN default.temp_right ri ON le.a=ri.a

左联测试结果

在右表中，b允许空，a和c冇允许空左联后，联不上的a是空字符串，联不上的b是NULL，联不上的c是0

INSERT SELECT

INSERT INTO t2
WITH a AS (SELECT * FROM t1)
SELECT * FROM a;

和HIVE、MySQL等不一样，ClickHouse的INSERT写在WITH之前

临时表

当回话结束时，临时表将随会话一起消失
临时表仅能用Memory表引擎
无法为临时表指定数据库，它是在数据库之外创建的
当查询没有指定库，且临时表与另一个表名相同时，会优先使用临时表

CREATE TEMPORARY TABLE temp_t(`a` String,`b` Int32);
INSERT INTO temp_t VALUES ('AB',3),('CC',4);
SELECT * FROM temp_t;
# 结束会话后，临时表不存在

窗口函数

CREATE TEMPORARY TABLE sales(
name  String  COMMENT '产品',
city  String  COMMENT '城市',
sale  Int32   COMMENT '销量');
INSERT INTO sales VALUES
('椰子','佛山',99),('雪梨','佛山',77),('苹果','佛山',88),
('椰子','广州',80),('雪梨','广州',80),('苹果','广州',70);

SELECT city
      ,groupArray(name) OVER (PARTITION BY city)
FROM sales;

SELECT
    city,
    name,
    sale,
    rank() OVER(PARTITION BY city ORDER BY sale DESC)
FROM sales;

单引号和双引号

多数情况使用单引号

SELECT "abc";
# 报错
SELECT 'abc';
# 正常查询，返回字符串

CREATE TABLE default.temp_t(
  `a` String COMMENT "匹配键"
)ENGINE=Log;
# 字段注释使用双引号报错
CREATE TABLE default.temp_t(
  `a` String COMMENT '匹配键'
)ENGINE=Log;
# 正常建表

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