query() for fast filtering in Pandas

Introduction

The query() method helps you quickly find rows in a table that match certain conditions. It makes filtering data easy and readable.

You want to select rows where a column value is greater than a number.

You need to filter data based on multiple conditions combined with AND or OR.

You want to write filtering code that looks clean and is easy to understand.

You are working with large tables and want a fast way to filter rows.

You prefer writing conditions as strings instead of using complex brackets.

Syntax

Pandas

DataFrame.query('condition')

The condition is a string that looks like a simple expression, e.g., 'age > 30'.

You can use column names directly inside the string without extra brackets.

Examples

Selects rows where the age column is greater than 25.

Pandas

df.query('age > 25')

Filters rows where the city column equals 'New York'.

Pandas

df.query('city == "New York"')

Filters rows where age is over 20 and city is 'Chicago'.

Pandas

df.query('age > 20 and city == "Chicago"')

Selects rows where score is at least 80 or grade is 'A'.

Pandas

df.query('score >= 80 or grade == "A"')

Sample Program

This code creates a small table of people with their ages and cities. It then uses query() to find people older than 30, and separately, people in New York who are younger than 30.

Pandas

import pandas as pd

data = {
    'name': ['Alice', 'Bob', 'Charlie', 'David'],
    'age': [25, 32, 18, 47],
    'city': ['New York', 'Chicago', 'New York', 'Chicago']
}
df = pd.DataFrame(data)

# Filter people older than 30
older_than_30 = df.query('age > 30')
print(older_than_30)

# Filter people in New York younger than 30
ny_young = df.query('city == "New York" and age < 30')
print(ny_young)

Important Notes

Use double quotes inside the query string if your values are strings, like city == "New York".

query() can be faster than normal filtering with brackets for large data.

Column names with spaces or special characters need backticks, e.g., df.query('`my column` > 5').

Summary

query() lets you filter rows using simple, readable strings.

It works well for conditions with one or more columns combined with and / or.

It can make your code cleaner and sometimes faster for big tables.

Practice

1. What is the main purpose of the query() method in pandas?

easy

A. To filter rows of a DataFrame using a readable string expression

B. To sort the DataFrame by column values

C. To merge two DataFrames based on a key column

D. To create a new DataFrame from scratch

Solution

1. Step 1: Understand the purpose of query()

   The query() method is designed to filter rows in a DataFrame by using a string expression that specifies conditions on columns.

2. Step 2: Compare with other DataFrame methods

   Sorting is done by sort_values(), merging by merge(), and creating new DataFrames is done by constructors, not query().

3. Final Answer:

   To filter rows of a DataFrame using a readable string expression -> Option A

4. Quick Check:

   query() filters rows [OK]

Hint: Remember: query() filters rows with string conditions [OK]

Common Mistakes:

• Confusing query() with sorting or merging methods
• Thinking query() creates new DataFrames
• Assuming query() modifies columns

2. Which of the following is the correct syntax to filter rows where column age is greater than 30 using query()?

easy

A. df.query('age > "30"')

B. df.query(age > 30)

C. df.query('age > 30')

D. df.query('age => 30')

Solution

1. Step 1: Check the correct string format for query()

   The condition must be passed as a string, so it should be inside quotes: 'age > 30'.

2. Step 2: Validate the comparison operator and quotes

   Operator must be >, not =>. Also, 30 should be a number, not a string with quotes.

3. Final Answer:

   df.query('age > 30') -> Option C

4. Quick Check:

   Use string with correct operator [OK]

Hint: Put condition inside quotes with correct operators [OK]

Common Mistakes:

• Omitting quotes around the condition string
• Using wrong comparison operator => instead of >
• Putting numbers inside quotes making them strings

3. Given the DataFrame:

import pandas as pd
df = pd.DataFrame({'name': ['Anna', 'Bob', 'Cara'], 'score': [85, 92, 78]})

What is the output of df.query('score >= 80')?

medium

A. Rows with names Anna and Bob only

B. Rows with names Bob and Cara only

C. Rows with names Anna and Cara only

D. All rows

Solution

1. Step 1: Identify rows where score >= 80

   Scores are Anna:85, Bob:92, Cara:78. Only Anna and Bob have scores 80 or above.

2. Step 2: Understand query() filters these rows

   The query returns rows where the condition is true, so rows for Anna and Bob are returned.

3. Final Answer:

   Rows with names Anna and Bob only -> Option A

4. Quick Check:

   score >= 80 filters Anna and Bob [OK]

Hint: Check numeric condition matches rows carefully [OK]

Common Mistakes:

• Including rows that don't meet the condition
• Confusing >= with > or <= operators
• Assuming query returns all rows

4. What is wrong with this code snippet?

df.query('age > 25 and city == New York')

medium

A. The column names must be in quotes

B. The operator 'and' is not allowed in query()

C. The query() method cannot filter by multiple conditions

D. The string New York should be quoted inside the query string

Solution

1. Step 1: Check string literals inside query()

   String values like city names must be quoted inside the query string, e.g., city == "New York".

2. Step 2: Verify use of 'and' and column names

   The 'and' operator is allowed, and column names do not need quotes inside the query string.

3. Final Answer:

   The string New York should be quoted inside the query string -> Option D

4. Quick Check:

   Quote string values inside query() [OK]

Hint: Quote string values inside query() conditions [OK]

Common Mistakes:

• Not quoting string literals in conditions
• Thinking 'and' is invalid in query()
• Quoting column names unnecessarily

5. You have a DataFrame df with columns temperature and humidity. You want to filter rows where temperature is above 20 and humidity is below 50. Which query() call correctly does this?

hard

A. df.query('temperature > 20 or humidity < 50')

B. df.query('temperature > 20 and humidity < 50')

C. df.query('temperature > 20 & humidity < 50')

D. df.query('temperature > 20 && humidity < 50')

Solution

1. Step 1: Understand logical operators in query()

   Within query strings, use and and or for combining conditions, not & or &&.

2. Step 2: Match the condition requirements

   The problem wants temperature > 20 and humidity < 50, so the correct operator is and.

3. Final Answer:

   df.query('temperature > 20 and humidity < 50') -> Option B

4. Quick Check:

   Use 'and' for multiple conditions in query() [OK]

Hint: Use 'and' not '&' to combine conditions in query() [OK]

Common Mistakes:

• Using bitwise operators (&, &&) instead of 'and'
• Using 'or' instead of 'and' when both conditions needed
• Mixing Python operators outside query() syntax