1. Core Navigation & Field Selection Filters

The simplest `jq` expression is the identity filter `.`, which pretty-prints incoming JSON. To extract specific keys, append property names after dots (e.g. `.user.email`).

Basic Property Extraction Queries 
# Sample JSON: {"user": {"name": "Alex", "email": "alex@example.com"}, "active": true}

# 1. Extract nested key
echo '{"user": {"name": "Alex", "email": "alex@example.com"}}' | jq '.user.email'
# Output: "alex@example.com"

# 2. Extract raw unquoted string (with -r flag)
echo '{"user": {"name": "Alex"}}' | jq -r '.user.name'
# Output: Alex
Explanation: The `-r` (raw-output) flag strips JSON double quotes around returned string values.

2. Array Iteration & Slicing (`.[]`)

To iterate over array items, use the array value iterator `.[]`. Combine it with index brackets to extract specific array slices or element positions.

Array Filtering & Mapping 
# Sample Array: [{"id": 1, "name": "A"}, {"id": 2, "name": "B"}]

# 1. Extract first element in array
echo '[{"name": "Alice"}, {"name": "Bob"}]' | jq '.[0]'

# 2. Extract all names across array items into a flat list
echo '[{"name": "Alice"}, {"name": "Bob"}]' | jq '.[].name'
# Output:
# "Alice"
# "Bob"
Explanation: `.[].name` iterates over every array item and yields its `name` property value.

3. Advanced Filtering with `select()` & Conditions

The `select(boolean_expression)` function filters JSON objects based on conditional evaluation (like SQL WHERE clauses).

Conditional Selection & Construction 
# Filter users where age > 25 and status is "active"
cat users.json | jq '.[] | select(.age > 25 and .status == "active")'

# Reconstruct new customized JSON object structure
cat users.json | jq '.[] | select(.active == true) | {username: .name, user_email: .email}'
Explanation: `select()` discards objects evaluating to false and passes matching items down the pipeline.