文本处理函数与时间日期处理函数的用法

1 文本处理函数

1.1 显示表格

SELECT * FROM student_info;

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1.2 LEFT() 返回从左边取指定长度的子串

SELECT LEFT(name, 1) FROM student_info;

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1.3 RIGHT() 返回从右边取指定长度的子串

SELECT RIGHT(name, 1) FROM student_info;

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1.4 LENGTH() 返回字符串的长度

SELECT LENGTH(name) FROM student_info;

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1.5 UPPER() 返回大写格式字符串

先新建一个示例表格：

CREATE TABLE example(
    id INT NOT NULL,
    info VARCHAR(64) NOT NULL
);
INSERT example VALUE 
(1, 'abc'),
(2, 'ABC'),
(3, '   ABC'),
(4, 'ABC  '),
(5, '  ABC  '),
(6, 'abcdefg');
SELECT * FROM example;

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SELECT UPPER(info) FROM example WHERE id = 1;

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1.6 LOWER() 返回小写格式字符串

SELECT LOWER(info) FROM example WHERE id = 2;

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1.7 LTRIM() 将字符串左边空格去除后返回

SELECT LTRIM(LOWER(INFO)) FROM example WHERE id = 3;

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1.8 RTRIM() 将字符串右边空格去除后返回

SELECT RTRIM(LOWER(INFO)) FROM example WHERE ID = 4;

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1.9 TRIM() 将字符串左右两边的空格去除后返回

SELECT TRIM(LOWER(INFO)) FROM example WHERE ID = 5;

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1.10 SUBSTRING() 从字符串第X位开始截取X位字符

SELECT SUBSTRING(INFO, 2, 2) FROM example WHERE id = 6;

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2 时间日期处理函数

2.1 NOW() 返回当前的日期和时间

SELECT NOW();

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2.2 CURDATE() 返回当前的日期

SELECT CURDATE();

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2.3 CURTIME() 返回当前的时间

SELECT CURTIME();

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2.4 EXTRACT() 返回日期/时间按的单独部分

SELECT EXTRACT(YEAR FROM NOW());

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SELECT EXTRACT(MONTH FROM NOW());

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2.5 DATE_ADD() 给日期添加指定的时间间隔

SELECT DATE_ADD(NOW(), INTERVAL 1 YEAR);

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2.6 DATE_SUB() 从日期减去指定的时间间隔

SELECT DATE_SUB(NOW(), INTERVAL 1 YEAR);

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2.7 DATEDIFF() 返回两个日期之间的天数

SELECT DATEDIFF('2019-02-28', '2020-02-29');

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2.8 DATE_FORMAT() 用不同的格式显示日期/时间

SELECT DATE_FORMAT('2020-02-29', '%Y年%m月%d日');

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