Mysql 串行执行nodejs

Question

mysqldb.js

const mysqlDatabasePool = mysql.createPool({
    connectionLimit: 50,
    host: process.env.MYSQL_HOST,
    user: process.env.MYSQL_USERNAME,
    password: process.env.MYSQL_PASSWORD,
    port: process.env.MYSQL_PORT,
    database: process.env.MYSQL_DATABASE
});

mysqlDatabasePool.getConnection({
  //Basic sanity checking code here
});

mysqlDatabasePool.query = util.promisify(mysqlDatabasePool.query);

module.exports = mysqlDatabasePool;

employeeController.js

var pool = require("mysqldb");

pool.query("SELECT CategoryID, CategoryIcon FROM Categories WHERE CategoryName = ?", [categoryName], function(error, results, fields) {
    if(results !== null && results.length > 0) {

        for(var i = 0; i < results.length; i++) {
             outputData[i] = results[i];

             var subCategories = pool.query("SELECT SubcategoryID FROM Subcategories WHERE CategoryID = ?", [results[i].CategoryID], function(error, results, fields) {
                 return results;
             });

             if(subCategories.length > 0) 
                  outputData[i]["Subcategories"] = subCategories;
             else
                  outputData[i]["Subcategories"] = null;
        }
    }
});

outputData[i]["Subcategories"] 的内容总是 null 而它应该有子类别的集合，因为它总是可用的。我尝试通过将 await 放在第二个 pool.query 之前并使第一个 pool.query 闭包函数异步来实现 await/async 。

应该如何处理？

Answer 1

作为参数传递给 pool.query 的函数是回调。这意味着您无法知道它何时会被执行。

您可以使用 async/await 来阻止调用，直到 pool.query 给您一个结果。

您可能需要在 pool.query 上使用 util.promisify 以使其 return 成为 Promise。

try {
   const results = await pool.query("SELECT CategoryID, CategoryIcon FROM Categories WHERE CategoryName = ?", [categoryName]);
   if(results !== null && results.length > 0) {

        for(var i = 0; i < results.length; i++) {
             outputData[i] = results[i];

             const subCategories = await pool.query("SELECT SubcategoryID FROM Subcategories WHERE CategoryID = ?", [results[i].CategoryID]);

             if(subCategories.length > 0) 
                  outputData[i]["Subcategories"] = subCategories;
             else
                  outputData[i]["Subcategories"] = null;
        }
    }
} catch(error) {
  console.log(error)
  // Deal with the error
}

以上是如何使用 async/await 语法的示例。记得使用关键字await，周围的函数必须标记为async.

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