The answer is to check the credentials used in the Source step. This is correct because the 'DataSource.Error: Microsoft SQL: Login failed for user' error is an authentication failure that occurs at the very first connection point in Power Query, where the system attempts to validate the user identity against the SQL Server database. Since the Source step is where the initial handshake and credential verification happen, no subsequent transformations—such as filtering rows or changing data types—can execute if this connection fails, making credential review the logical first step in any Power Query SQL login failed error troubleshoot. On the PL-300 exam, this tests your understanding of Power Query’s query execution order, where the Source step is the root of all data retrieval; a common trap is to look for issues in later applied steps, but the error message directly points to authentication, not data shaping. Memory tip: “Source first, fix the worst”—always verify credentials at the Source step before troubleshooting downstream transformations.
PL-300 Prepare the data Practice Question
This PL-300 practice question tests your understanding of prepare the data. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
Refer to the exhibit.
// Power Query M code
let
Source = Sql.Database("server", "database"),
dbo_Orders = Source{[Schema="dbo",Item="Orders"]}[Data],
#"Filtered Rows" = Table.SelectRows(dbo_Orders, each [OrderDate] > #date(2024,1,1)),
#"Removed Columns" = Table.RemoveColumns(#"Filtered Rows",{"CreditCardNumber"}),
#"Changed Type" = Table.TransformColumnTypes(#"Removed Columns",{{"OrderDate", type date}})
in
#"Changed Type"
Refer to the exhibit. You are reviewing a Power Query script. The script fails with a 'DataSource.Error: Microsoft SQL: Login failed for user' error. Which step should you check first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Refer to the exhibit.
// Power Query M code
let
Source = Sql.Database("server", "database"),
dbo_Orders = Source{[Schema="dbo",Item="Orders"]}[Data],
#"Filtered Rows" = Table.SelectRows(dbo_Orders, each [OrderDate] > #date(2024,1,1)),
#"Removed Columns" = Table.RemoveColumns(#"Filtered Rows",{"CreditCardNumber"}),
#"Changed Type" = Table.TransformColumnTypes(#"Removed Columns",{{"OrderDate", type date}})
in
#"Changed Type"
A
The column name in #"Removed Columns".
Why wrong: Not related to login.
B
The data type transformation in #"Changed Type".
Why wrong: Not related to login.
C
The filter condition in #"Filtered Rows".
Why wrong: This step runs after successful connection.
D
The credentials used in the Source step.
Login failed indicates the credentials are incorrect or insufficient.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The credentials used in the Source step.
The error 'DataSource.Error: Microsoft SQL: Login failed for user' indicates an authentication failure when connecting to the SQL Server database. This occurs at the Source step, where Power Query first attempts to establish a connection using the provided credentials. Checking and correcting the credentials in the Source step is the logical first step because no subsequent data transformations (like removing columns, changing types, or filtering rows) can execute if the initial data source connection fails.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The column name in #"Removed Columns".
Why it's wrong here
Not related to login.
✗
The data type transformation in #"Changed Type".
Why it's wrong here
Not related to login.
✗
The filter condition in #"Filtered Rows".
Why it's wrong here
This step runs after successful connection.
✓
The credentials used in the Source step.
Why this is correct
Login failed indicates the credentials are incorrect or insufficient.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may focus on data transformation steps (like removing columns or changing types) because they appear later in the query, but the error originates at the very first step—the Source step—where authentication is validated before any data is retrieved.
Detailed technical explanation
How to think about this question
Power Query's Source step uses a connection string and stored credentials (Windows Authentication or SQL Server Authentication) to open a TCP/IP connection to the SQL Server instance. The login failure error (error code 18456) is raised by SQL Server's security layer before any T-SQL query is executed, meaning the M expression in the Source step fails immediately. In real-world scenarios, this often occurs when credentials are expired, the user lacks CONNECT permission, or the SQL Server is configured for Windows Authentication only but Power Query is using SQL login.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Prepare the data — This question tests Prepare the data — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The credentials used in the Source step. — The error 'DataSource.Error: Microsoft SQL: Login failed for user' indicates an authentication failure when connecting to the SQL Server database. This occurs at the Source step, where Power Query first attempts to establish a connection using the provided credentials. Checking and correcting the credentials in the Source step is the logical first step because no subsequent data transformations (like removing columns, changing types, or filtering rows) can execute if the initial data source connection fails.
What should I do if I get this PL-300 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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