WHERE 子句
描述
WHERE
子句用于根据指定条件限制查询或子查询的
FROM
子句的结果。
语法
WHERE 布尔表达式
参数
-
boolean_expression
指定任何评估为结果类型
boolean的表达式。可以使用逻辑运算符（AND，OR）将两个或更多表达式组合在一起。
示例
CREATE TABLE person (id INT, name STRING, age INT);
INSERT INTO person VALUES
(100, 'John', 30),
(200, 'Mary', NULL),
(300, 'Mike', 80),
(400, 'Dan', 50);
-- `WHERE`子句中的比较运算符。
SELECT * FROM person WHERE id > 200 ORDER BY id;
+---+----+---+
| id|name|age|
+---+----+---+
|300|Mike| 80|
|400| Dan| 50|
+---+----+---+
-- `WHERE`子句中的比较和逻辑运算符。
SELECT * FROM person WHERE id = 200 OR id = 300 ORDER BY id;
+---+----+----+
| id|name| age|
+---+----+----+
|200|Mary|null|
|300|Mike| 80|
+---+----+----+
-- `WHERE`子句中的IS NULL表达式。
SELECT * FROM person WHERE id > 300 OR age IS NULL ORDER BY id;
+---+----+----+
| id|name| age|
+---+----+----+
|200|Mary|null|
|400| Dan| 50|
+---+----+----+
-- `WHERE`子句中的函数表达式。
SELECT * FROM person WHERE length(name) > 3 ORDER BY id;
+---+----+----+
| id|name| age|
+---+----+----+
|100|John| 30|
|200|Mary|null|
|300|Mike| 80|
+---+----+----+
-- `WHERE`子句中的`BETWEEN`表达式。
SELECT * FROM person WHERE id BETWEEN 200 AND 300 ORDER BY id;
+---+----+----+
| id|name| age|
+---+----+----+
|200|Mary|null|
|300|Mike| 80|
+---+----+----+
-- `WHERE`子句中的标量子查询。
SELECT * FROM person WHERE age > (SELECT avg(age) FROM person);
+---+----+---+
| id|name|age|
+---+----+---+
|300|Mike| 80|
+---+----+---+
-- `WHERE`子句中的相关子查询。
SELECT * FROM person AS parent
WHERE EXISTS (
SELECT 1 FROM person AS child
WHERE parent.id = child.id AND child.age IS NULL
);
+---+----+----+
|id |name|age |
+---+----+----+
|200|Mary|null|
+---+----+----+