# Querying Fields (Python)
Learn to use MRRC's Query DSL for complex field searching.
## Basic Field Access
```python
import mrrc
for record in mrrc.MARCReader("records.mrc"):
# Get all fields with a tag
for field in record.fields_by_tag("650"):
print(field["a"])
```
## Filter by Indicators
Find fields with specific indicator values:
```python
# Find 650 fields with indicator2='0' (LCSH)
lcsh_subjects = record.fields_by_indicator("650", indicator2="0")
for field in lcsh_subjects:
print(field["a"])
# Match both indicators
fields = record.fields_by_indicator("245", indicator1="1", indicator2="0")
```
## Filter by Tag Range
Find fields within a tag range:
```python
# Find all subject fields (600-699)
subjects = record.fields_in_range("600", "699")
for field in subjects:
print(f"{field.tag}: {field['a']}")
# Find all notes (500-599)
notes = record.fields_in_range("500", "599")
```
## FieldQuery (Complex Matching)
Build complex queries with FieldQuery:
```python
# Create a query builder
query = mrrc.FieldQuery()
query.tag("650")
query.indicator2("0")
query.has_subfield("a")
# Execute the query
for field in record.fields_matching(query):
print(field["a"])
```
### Combining Conditions
```python
query = mrrc.FieldQuery()
query.tag("650") # Must be 650
query.indicator2("0") # Indicator2 = 0
query.has_subfield("a") # Must have $a
query.has_subfield("x") # Must have $x subdivision
matches = record.fields_matching(query)
```
## Pattern Matching
Match subfield values with regular expressions:
```python
# Find ISBN-13s (start with 978 or 979)
query = mrrc.SubfieldPatternQuery("020", "a", r"^97[89]")
for field in record.fields_matching_pattern(query):
print(field["a"])
# Find emails in 856 field
query = mrrc.SubfieldPatternQuery("856", "u", r"@.*\.")
for field in record.fields_matching_pattern(query):
print(field["u"])
# Negated: find ISBNs that are NOT ISBN-13
query = mrrc.SubfieldPatternQuery("020", "a", r"^97[89]", negate=True)
for field in record.fields_matching_pattern(query):
print(field["a"])
```
## Value Matching
Match exact or partial subfield values:
```python
# Exact match
query = mrrc.SubfieldValueQuery("650", "a", "History")
exact_matches = record.fields_matching_value(query)
# Partial match (contains)
query = mrrc.SubfieldValueQuery("650", "a", "History", partial=True)
partial_matches = record.fields_matching_value(query)
# Negated: find subjects that are NOT "History"
query = mrrc.SubfieldValueQuery("650", "a", "History", negate=True)
non_history = record.fields_matching_value(query)
```
## Complete Example
```python
#!/usr/bin/env python3
"""Find records with specific subject headings."""
import mrrc
def find_lcsh_subjects(path, search_term):
"""Find records with LCSH subjects containing a term."""
results = []
for record in mrrc.MARCReader(path):
# Query for LCSH subjects (indicator2=0) containing search term
query = mrrc.SubfieldValueQuery("650", "a", search_term, partial=True)
# Also check indicator2 = 0 (LCSH)
for field in record.fields_by_indicator("650", indicator2="0"):
if search_term.lower() in (field["a"] or "").lower():
results.append({
"title": record.title,
"subject": field["a"]
})
return results
# Find records about "computers"
results = find_lcsh_subjects("library.mrc", "computers")
for r in results:
print(f"{r['title']}: {r['subject']}")
```
## Performance Tips
- Use specific tags when possible (faster than ranges)
- Filter by indicator first, then by subfield content
- For large files, consider using the concurrency features
## Next Steps
- [Reading Records](reading-records.md) - Basic record access
- [Concurrency](concurrency.md) - Parallel processing
- [Query DSL Guide](../../guides/query-dsl.md) - Full Query DSL reference