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Look at the example code like this;

const mysql = require('mysql');

exports.base = (sql, data, callback) => {
    // 創(chuàng)建數(shù)據(jù)庫(kù)連接
    let connection = mysql.createConnection({
        host: 'localhost', //數(shù)據(jù)庫(kù)所在的服務(wù)器域名或者IP
        user: 'root', //用戶名
        password: '', //密碼
        database: 'book' //數(shù)據(jù)庫(kù)名稱
    });
    // 執(zhí)行連接動(dòng)作
    connection.connect();
    // 執(zhí)行數(shù)據(jù)庫(kù)操作
    connection.query(sql, data, (err, rows) => {
        if (err) throw err;
        callback(rows);
    });
    // 關(guān)閉數(shù)據(jù)庫(kù)
    connection.end();
}

It should look like this

const mysql = require('mysql');

exports.base = (sql, data, callback) => {
    // 創(chuàng)建數(shù)據(jù)庫(kù)連接
    let connection = mysql.createConnection({
        host: 'localhost', //數(shù)據(jù)庫(kù)所在的服務(wù)器域名或者IP
        user: 'root', //用戶名
        password: '', //密碼
        database: 'book' //數(shù)據(jù)庫(kù)名稱
    });
    // 執(zhí)行連接動(dòng)作
    connection.connect();
    // 執(zhí)行數(shù)據(jù)庫(kù)操作
    connection.query(sql, data, (err, rows) => {
        if (err) throw err;
        callback(rows);
        // 關(guān)閉數(shù)據(jù)庫(kù)
        connection.end();
    });
    
}

I just feel that closing the database connection should be done in the callback of the query. If it is written like the first way and the query is not finished, is it inappropriate to close the database? The internal principle of this mysql module is not very clear;
Hope everyone can clear up the confusion;