{"id":1810,"date":"2025-02-18T23:09:25","date_gmt":"2025-02-18T15:09:25","guid":{"rendered":"https:\/\/www.laixuexila.com\/?p=1810"},"modified":"2025-02-18T23:09:25","modified_gmt":"2025-02-18T15:09:25","slug":"mysql-%e5%87%bd%e6%95%b0","status":"publish","type":"post","link":"https:\/\/www.laixuexila.com\/index.php\/2025\/02\/18\/mysql-%e5%87%bd%e6%95%b0\/","title":{"rendered":"MySQL \u51fd\u6570"},"content":{"rendered":"\n<p>MySQL \u63d0\u4f9b\u4e86\u4e30\u5bcc\u7684\u5185\u7f6e\u51fd\u6570\uff0c\u4f9b\u5f00\u53d1\u8005\u5728 SQL \u67e5\u8be2\u4e2d\u4f7f\u7528\u3002MySQL \u51fd\u6570\u53ef\u4ee5\u5206\u4e3a\u4e0d\u540c\u7c7b\u578b\uff0c\u5982\u5b57\u7b26\u4e32\u51fd\u6570\u3001\u6570\u5b66\u51fd\u6570\u3001\u65e5\u671f\u548c\u65f6\u95f4\u51fd\u6570\u3001\u805a\u5408\u51fd\u6570\u7b49\u3002\u4ee5\u4e0b\u662f\u4e00\u4e9b\u5e38\u89c1\u7684 MySQL \u51fd\u6570\u53ca\u5176\u5b9e\u4f8b\uff0c\u5e2e\u52a9\u4f60\u6df1\u5165\u7406\u89e3\u5982\u4f55\u4f7f\u7528\u5b83\u4eec\u3002<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">1. <strong>\u5b57\u7b26\u4e32\u51fd\u6570<\/strong><\/h3>\n\n\n\n<h4 class=\"wp-block-heading\">1.1 <code>CONCAT()<\/code><\/h4>\n\n\n\n<p><code>CONCAT()<\/code> \u51fd\u6570\u7528\u4e8e\u8fde\u63a5\u591a\u4e2a\u5b57\u7b26\u4e32\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT CONCAT('Hello', ' ', 'World') AS greeting;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>greeting\n----------------\nHello World<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">1.2 <code>LENGTH()<\/code><\/h4>\n\n\n\n<p><code>LENGTH()<\/code> \u51fd\u6570\u8fd4\u56de\u5b57\u7b26\u4e32\u7684\u5b57\u8282\u6570\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT LENGTH('Hello') AS length;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>length\n------\n5<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">1.3 <code>UPPER()<\/code> \/ <code>LOWER()<\/code><\/h4>\n\n\n\n<p><code>UPPER()<\/code> \u5c06\u5b57\u7b26\u4e32\u8f6c\u6362\u4e3a\u5927\u5199\uff0c<code>LOWER()<\/code> \u5c06\u5b57\u7b26\u4e32\u8f6c\u6362\u4e3a\u5c0f\u5199\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT UPPER('hello') AS uppercase, LOWER('HELLO') AS lowercase;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>uppercase  | lowercase\n-----------|----------\nHELLO      | hello<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">1.4 <code>TRIM()<\/code><\/h4>\n\n\n\n<p><code>TRIM()<\/code> \u53bb\u6389\u5b57\u7b26\u4e32\u4e24\u7aef\u7684\u7a7a\u683c\u6216\u5176\u4ed6\u5b57\u7b26\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT TRIM(' ' FROM '  Hello  ') AS trimmed;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>trimmed\n-------\nHello<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">1.5 <code>SUBSTRING()<\/code><\/h4>\n\n\n\n<p><code>SUBSTRING()<\/code> \u51fd\u6570\u8fd4\u56de\u5b57\u7b26\u4e32\u7684\u5b50\u4e32\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT SUBSTRING('Hello World', 1, 5) AS substring;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>substring\n----------\nHello<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">2. <strong>\u6570\u5b66\u51fd\u6570<\/strong><\/h3>\n\n\n\n<h4 class=\"wp-block-heading\">2.1 <code>ROUND()<\/code><\/h4>\n\n\n\n<p><code>ROUND()<\/code> \u51fd\u6570\u7528\u4e8e\u5bf9\u6570\u5b57\u8fdb\u884c\u56db\u820d\u4e94\u5165\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT ROUND(123.456, 2) AS rounded_value;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>rounded_value\n--------------\n123.46<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">2.2 <code>FLOOR()<\/code> \u548c <code>CEIL()<\/code><\/h4>\n\n\n\n<p><code>FLOOR()<\/code> \u8fd4\u56de\u5c0f\u4e8e\u6216\u7b49\u4e8e\u6307\u5b9a\u6570\u5b57\u7684\u6700\u5927\u6574\u6570\uff0c<code>CEIL()<\/code> \u8fd4\u56de\u5927\u4e8e\u6216\u7b49\u4e8e\u6307\u5b9a\u6570\u5b57\u7684\u6700\u5c0f\u6574\u6570\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT FLOOR(123.456) AS floor_value, CEIL(123.456) AS ceil_value;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>floor_value | ceil_value\n------------|------------\n123         | 124<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">2.3 <code>ABS()<\/code><\/h4>\n\n\n\n<p><code>ABS()<\/code> \u8fd4\u56de\u6570\u5b57\u7684\u7edd\u5bf9\u503c\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT ABS(-123) AS absolute_value;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>absolute_value\n---------------\n123<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">2.4 <code>MOD()<\/code><\/h4>\n\n\n\n<p><code>MOD()<\/code> \u8fd4\u56de\u4e24\u4e2a\u6570\u5b57\u76f8\u9664\u7684\u4f59\u6570\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT MOD(10, 3) AS remainder;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>remainder\n----------\n1<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">3. <strong>\u65e5\u671f\u548c\u65f6\u95f4\u51fd\u6570<\/strong><\/h3>\n\n\n\n<h4 class=\"wp-block-heading\">3.1 <code>NOW()<\/code> \u548c <code>CURDATE()<\/code><\/h4>\n\n\n\n<p><code>NOW()<\/code> \u8fd4\u56de\u5f53\u524d\u7684\u65e5\u671f\u548c\u65f6\u95f4\uff0c<code>CURDATE()<\/code> \u8fd4\u56de\u5f53\u524d\u7684\u65e5\u671f\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT NOW() AS current_datetime, CURDATE() AS current_date;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>current_datetime        | current_date\n------------------------|--------------\n2025-02-18 10:00:00    | 2025-02-18<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">3.2 <code>DATE_ADD()<\/code> \u548c <code>DATE_SUB()<\/code><\/h4>\n\n\n\n<p><code>DATE_ADD()<\/code> \u7528\u4e8e\u7ed9\u65e5\u671f\u52a0\u4e0a\u6307\u5b9a\u7684\u65f6\u95f4\u95f4\u9694\uff0c<code>DATE_SUB()<\/code> \u7528\u4e8e\u4ece\u65e5\u671f\u4e2d\u51cf\u53bb\u6307\u5b9a\u7684\u65f6\u95f4\u95f4\u9694\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT DATE_ADD('2025-02-18', INTERVAL 5 DAY) AS date_added,\n       DATE_SUB('2025-02-18', INTERVAL 5 DAY) AS date_subtracted;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>date_added | date_subtracted\n-----------|----------------\n2025-02-23 | 2025-02-13<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">3.3 <code>DATEDIFF()<\/code><\/h4>\n\n\n\n<p><code>DATEDIFF()<\/code> \u51fd\u6570\u8fd4\u56de\u4e24\u4e2a\u65e5\u671f\u4e4b\u95f4\u7684\u5929\u6570\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT DATEDIFF('2025-02-18', '2025-02-10') AS date_diff;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>date_diff\n----------\n8<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">3.4 <code>YEAR()<\/code>, <code>MONTH()<\/code>, <code>DAY()<\/code><\/h4>\n\n\n\n<p>\u8fd9\u4e9b\u51fd\u6570\u5206\u522b\u63d0\u53d6\u65e5\u671f\u7684\u5e74\u4efd\u3001\u6708\u4efd\u548c\u65e5\u671f\u90e8\u5206\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT YEAR('2025-02-18') AS year,\n       MONTH('2025-02-18') AS month,\n       DAY('2025-02-18') AS day;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>year | month | day\n-----|-------|-----\n2025 | 2     | 18<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">4. <strong>\u805a\u5408\u51fd\u6570<\/strong><\/h3>\n\n\n\n<h4 class=\"wp-block-heading\">4.1 <code>COUNT()<\/code><\/h4>\n\n\n\n<p><code>COUNT()<\/code> \u51fd\u6570\u8fd4\u56de\u67e5\u8be2\u7ed3\u679c\u4e2d\u884c\u7684\u6570\u91cf\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT COUNT(*) AS total_employees FROM employees;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>total_employees\n----------------\n100<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">4.2 <code>SUM()<\/code><\/h4>\n\n\n\n<p><code>SUM()<\/code> \u51fd\u6570\u8fd4\u56de\u5217\u4e2d\u6240\u6709\u503c\u7684\u603b\u548c\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT SUM(salary) AS total_salary FROM employees;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>total_salary\n-------------\n500000<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">4.3 <code>AVG()<\/code><\/h4>\n\n\n\n<p><code>AVG()<\/code> \u51fd\u6570\u8fd4\u56de\u5217\u4e2d\u6240\u6709\u503c\u7684\u5e73\u5747\u503c\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT AVG(salary) AS average_salary FROM employees;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>average_salary\n---------------\n5000<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">4.4 <code>MAX()<\/code> \u548c <code>MIN()<\/code><\/h4>\n\n\n\n<p><code>MAX()<\/code> \u51fd\u6570\u8fd4\u56de\u5217\u4e2d\u7684\u6700\u5927\u503c\uff0c<code>MIN()<\/code> \u51fd\u6570\u8fd4\u56de\u5217\u4e2d\u7684\u6700\u5c0f\u503c\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT MAX(salary) AS max_salary, MIN(salary) AS min_salary FROM employees;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>max_salary | min_salary\n-----------|-----------\n10000      | 3000<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">5. <strong>\u6761\u4ef6\u51fd\u6570<\/strong><\/h3>\n\n\n\n<h4 class=\"wp-block-heading\">5.1 <code>IF()<\/code><\/h4>\n\n\n\n<p><code>IF()<\/code> \u51fd\u6570\u6839\u636e\u6761\u4ef6\u8fd4\u56de\u4e0d\u540c\u7684\u503c\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT IF(salary &gt; 5000, 'High', 'Low') AS salary_level\nFROM employees;<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">5.2 <code>CASE<\/code><\/h4>\n\n\n\n<p><code>CASE<\/code> \u662f SQL \u4e2d\u7684\u6761\u4ef6\u8868\u8fbe\u5f0f\uff0c\u53ef\u4ee5\u7528\u4e8e\u591a\u4e2a\u6761\u4ef6\u7684\u5224\u65ad\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT \n    CASE \n        WHEN salary &gt; 8000 THEN 'High'\n        WHEN salary &gt; 5000 THEN 'Medium'\n        ELSE 'Low'\n    END AS salary_level\nFROM employees;<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">6. <strong>\u7c7b\u578b\u8f6c\u6362\u51fd\u6570<\/strong><\/h3>\n\n\n\n<h4 class=\"wp-block-heading\">6.1 <code>CAST()<\/code> \u548c <code>CONVERT()<\/code><\/h4>\n\n\n\n<p><code>CAST()<\/code> \u548c <code>CONVERT()<\/code> \u7528\u4e8e\u5c06\u4e00\u4e2a\u6570\u636e\u7c7b\u578b\u8f6c\u6362\u4e3a\u53e6\u4e00\u4e2a\u6570\u636e\u7c7b\u578b\u3002<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>SELECT CAST('123.45' AS UNSIGNED) AS cast_value,\n       CONVERT('123.45', SIGNED) AS convert_value;<\/code><\/pre>\n\n\n\n<p>\u8f93\u51fa\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>cast_value | convert_value\n-----------|--------------\n123        | 123<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">7. <strong>\u603b\u7ed3<\/strong><\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>\u5b57\u7b26\u4e32\u51fd\u6570<\/strong>\uff1a\u5982 <code>CONCAT()<\/code>\u3001<code>LENGTH()<\/code>\u3001<code>UPPER()<\/code> \u7b49\uff0c\u7528\u4e8e\u64cd\u4f5c\u5b57\u7b26\u4e32\u3002<\/li>\n\n\n\n<li><strong>\u6570\u5b66\u51fd\u6570<\/strong>\uff1a\u5982 <code>ROUND()<\/code>\u3001<code>FLOOR()<\/code>\u3001<code>ABS()<\/code> \u7b49\uff0c\u7528\u4e8e\u6570\u5b66\u8ba1\u7b97\u3002<\/li>\n\n\n\n<li><strong>\u65e5\u671f\u548c\u65f6\u95f4\u51fd\u6570<\/strong>\uff1a\u5982 <code>NOW()<\/code>\u3001<code>DATE_ADD()<\/code>\u3001<code>DATEDIFF()<\/code> \u7b49\uff0c\u7528\u4e8e\u64cd\u4f5c\u65e5\u671f\u548c\u65f6\u95f4\u3002<\/li>\n\n\n\n<li><strong>\u805a\u5408\u51fd\u6570<\/strong>\uff1a\u5982 <code>COUNT()<\/code>\u3001<code>SUM()<\/code>\u3001<code>AVG()<\/code> \u7b49\uff0c\u7528\u4e8e\u805a\u5408\u6570\u636e\u3002<\/li>\n\n\n\n<li><strong>\u6761\u4ef6\u51fd\u6570<\/strong>\uff1a\u5982 <code>IF()<\/code> \u548c <code>CASE<\/code>\uff0c\u7528\u4e8e\u6839\u636e\u6761\u4ef6\u8fd4\u56de\u4e0d\u540c\u7684\u7ed3\u679c\u3002<\/li>\n\n\n\n<li><strong>\u7c7b\u578b\u8f6c\u6362\u51fd\u6570<\/strong>\uff1a\u5982 <code>CAST()<\/code> \u548c <code>CONVERT()<\/code>\uff0c\u7528\u4e8e\u7c7b\u578b\u8f6c\u6362\u3002<\/li>\n<\/ul>\n\n\n\n<p>MySQL \u51fd\u6570\u63d0\u4f9b\u4e86\u5f3a\u5927\u7684\u6570\u636e\u5904\u7406\u80fd\u529b\uff0c\u53ef\u4ee5\u5728 SQL \u67e5\u8be2\u4e2d\u7075\u6d3b\u5730\u5904\u7406\u6570\u636e\u548c\u8ba1\u7b97\uff0c\u63d0\u5347\u67e5\u8be2\u6548\u7387\u548c\u53ef\u8bfb\u6027\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"<p>MySQL \u63d0\u4f9b\u4e86\u4e30\u5bcc\u7684\u5185\u7f6e\u51fd\u6570\uff0c\u4f9b\u5f00\u53d1\u8005\u5728 SQL \u67e5\u8be2\u4e2d\u4f7f\u7528\u3002MySQL \u51fd\u6570\u53ef\u4ee5\u5206\u4e3a\u4e0d\u540c\u7c7b\u578b\uff0c\u5982\u5b57\u7b26\u4e32\u51fd [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[35],"tags":[],"class_list":["post-1810","post","type-post","status-publish","format-standard","hentry","category-mysql"],"_links":{"self":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/1810","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/comments?post=1810"}],"version-history":[{"count":1,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/1810\/revisions"}],"predecessor-version":[{"id":1811,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/1810\/revisions\/1811"}],"wp:attachment":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/media?parent=1810"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/categories?post=1810"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/tags?post=1810"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}