Choose a data loading and movement pattern with mssql-python

The mssql-python driver provides multiple paths for writing data into Microsoft SQL. Each path fits different workloads. This guide helps you choose the right one based on your data volume, source format, and update semantics.

Decide by workload

Load CSV data with bulk copy

Loading CSV data is the most common ingest question for Python database work. Use csv.reader with a generator feeding bulkcopy():

import csv
import mssql_python

conn = mssql_python.connect(connection_string)
cursor = conn.cursor()

# Create a target table
cursor.execute("""
    IF NOT EXISTS (SELECT * FROM sys.tables WHERE name = 'ProductImport')
    CREATE TABLE dbo.ProductImport (
        Name nvarchar(100),
        ProductNumber nvarchar(25),
        ListPrice decimal(10,2)
    )
""")
conn.commit()

def csv_rows(path):
    with open(path, newline="", encoding="utf-8") as f:
        reader = csv.reader(f)
        next(reader)  # Skip header
        for row in reader:
            yield (row[0], row[1], float(row[2]))

result = cursor.bulkcopy(
    "dbo.ProductImport",
    csv_rows("products.csv"),
    batch_size=5000
)
print(f"Loaded {result['rows_copied']} rows")
conn.commit()

The generator pattern keeps memory usage constant regardless of file size. For column mapping and identity handling, see Bulk copy operations.

Single row inserts

Use single inserts for application-level writes where you process one record at a time. Use OUTPUT INSERTED to retrieve generated keys:

cursor.execute("""
    INSERT INTO dbo.ProductImport (Name, ProductNumber, ListPrice)
    OUTPUT INSERTED.Name
    VALUES (%(name)s, %(product_number)s, %(list_price)s)
""", {"name": "Widget", "product_number": "WG-1000", "list_price": 19.99})

inserted_name = cursor.fetchval()
conn.commit()

Single inserts are the right choice when:

• You insert one row per user action (form submission, API call).
• You need to validate or transform each row individually before inserting.
• You need the inserted ID or other generated values immediately.

Batched inserts

Use executemany() when you have a moderate number of rows and don't need bulk copy's throughput:

rows = [
    {"name": "Widget A", "product_number": "WG-1001", "list_price": 19.99},
    {"name": "Widget B", "product_number": "WG-1002", "list_price": 24.99},
    {"name": "Widget C", "product_number": "WG-1003", "list_price": 29.99},
]

cursor.executemany(
    "INSERT INTO dbo.ProductImport (Name, ProductNumber, ListPrice) VALUES (%(name)s, %(product_number)s, %(list_price)s)",
    rows
)
conn.commit()

executemany() sends each row as a separate parameterized statement. When throughput matters more than per-row control, bulkcopy() is more efficient because it uses the TDS bulk insert protocol. The crossover depends on row width and network latency, but it's typically in the low hundreds of rows.

Bulk copy

When throughput matters more than per-row control, use bulkcopy(). It uses the TDS bulk insert protocol, which is significantly more efficient than row-by-row inserts:

rows = [
    ("Widget A", "WG-1001", 19.99),
    ("Widget B", "WG-1002", 24.99),
    ("Widget C", "WG-1003", 29.99),
]

result = cursor.bulkcopy("dbo.ProductImport", rows, batch_size=5000)
print(f"Loaded {result['rows_copied']} rows")
conn.commit()

Performance tips for bulk copy

• Use generators for large datasets to keep memory usage constant.
• Set batch_size to control how many rows are sent per TDS batch. Start with 5,000 and adjust based on row width.
• Use table locks for exclusive loads: cursor.bulkcopy("dbo.ProductImport", rows, table_lock=True).
• Disable indexes before loading, then rebuild after. This sequence avoids index maintenance overhead during the load.

For column mappings, identity columns, NULL handling, and parallel loading, see Bulk copy operations.

Upsert with MERGE

MERGE is Microsoft SQL's statement for conditional INSERT, UPDATE, and DELETE in a single operation. It handles the "insert if new, update if exists" pattern that Python developers commonly need.

Single-row upsert

For a single row, use MERGE with a USING clause that defines parameter aliases:

cursor.execute("""
    MERGE dbo.ProductImport AS target
    USING (SELECT %(name)s AS Name, %(product_number)s AS ProductNumber, %(list_price)s AS ListPrice) AS source
    ON target.ProductNumber = source.ProductNumber
    WHEN MATCHED THEN
        UPDATE SET
            Name = source.Name,
            ListPrice = source.ListPrice
    WHEN NOT MATCHED THEN
        INSERT (Name, ProductNumber, ListPrice)
        VALUES (source.Name, source.ProductNumber, source.ListPrice);
""", {"name": "Widget A", "product_number": "WG-1001", "list_price": 24.99})
conn.commit()

Bulk upsert with a staging table

For bulk upserts, stage data into a temp table first, then use MERGE to update from it. Use insert-or-update as the default pattern for DataFrame upserts and batch updates:

import csv
import mssql_python

conn = mssql_python.connect(connection_string)
cursor = conn.cursor()

# Step 1: Create a global temp table for staging
# Note: bulkcopy() requires global temp tables (##), not session temp tables (#)
cursor.execute("""
    IF OBJECT_ID('tempdb..##ProductImportStage') IS NOT NULL
        DROP TABLE ##ProductImportStage;
    CREATE TABLE ##ProductImportStage (
        Name nvarchar(100),
        ProductNumber nvarchar(25),
        ListPrice decimal(10,2)
    )
""")
cursor.commit()

# Step 2: Bulk load into the staging table
def csv_rows(path):
    with open(path, newline="", encoding="utf-8") as f:
        reader = csv.reader(f)
        next(reader)
        for row in reader:
            yield (row[0], row[1], float(row[2]))

cursor.bulkcopy("##ProductImportStage", csv_rows("products_update.csv"), batch_size=5000)

# Step 3: MERGE from staging into the target table
cursor.execute("""
    MERGE dbo.ProductImport AS target
    USING ##ProductImportStage AS source
    ON target.ProductNumber = source.ProductNumber
    WHEN MATCHED THEN
        UPDATE SET
            Name = source.Name,
            ListPrice = source.ListPrice
    WHEN NOT MATCHED BY TARGET THEN
        INSERT (Name, ProductNumber, ListPrice)
        VALUES (source.Name, source.ProductNumber, source.ListPrice)
    OUTPUT $action, INSERTED.ProductNumber, DELETED.ProductNumber;
""")

# Step 4: Read the OUTPUT to see what changed
for row in cursor.fetchall():
    print(f"{row[0]}: inserted={row[1]}, deleted={row[2]}")

conn.commit()

This example demonstrates the default insert-or-update pattern:

• INSERT rows from the source that don't exist in the target (WHEN NOT MATCHED BY TARGET).
• UPDATE rows that exist in both (WHEN MATCHED).
• OUTPUT clause reports what action was taken on each row, which is useful for audit trails.

If you need full reconciliation, extend the MERGE only after you confirm the source is authoritative for the target table:

WHEN NOT MATCHED BY SOURCE THEN
    DELETE

In shared environments, use a unique global temp table name per run or a permanent staging table to avoid collisions between concurrent jobs.

When to use separate UPDATE and INSERT statements instead

MERGE is powerful but has edge cases. Consider using separate statements when:

• You don't need DELETE logic. A separate UPDATE followed by INSERT WHERE NOT EXISTS is more readable and straightforward to debug.
• The MERGE statement is complex enough that the locking behavior is hard to predict. Separate statements give you explicit control over lock granularity.
• You're updating a high-concurrency table where MERGE lock escalation could cause blocking.

# Simpler alternative: UPDATE then INSERT
cursor.execute("""
    UPDATE dbo.ProductImport
    SET Name = %(name)s, ListPrice = %(list_price)s
    WHERE ProductNumber = %(product_number)s
""", {"name": "Widget A", "list_price": 24.99, "product_number": "WG-1001"})

if cursor.rowcount == 0:
    cursor.execute("""
        INSERT INTO dbo.ProductImport (Name, ProductNumber, ListPrice)
        VALUES (%(name)s, %(product_number)s, %(list_price)s)
    """, {"name": "Widget A", "product_number": "WG-1001", "list_price": 24.99})

conn.commit()

Load DataFrames

Extract rows from a pandas or Polars DataFrame and load them by using bulkcopy():

pandas

Convert a pandas DataFrame to tuples and pass to bulkcopy():

import pandas as pd

df = pd.read_csv("products.csv")

# Convert DataFrame rows to tuples
rows = list(df[["Name", "ProductNumber", "ListPrice"]].itertuples(index=False, name=None))

cursor.bulkcopy("dbo.ProductImport", rows, batch_size=5000)
conn.commit()

Polars

Convert a Polars DataFrame to tuples using the .rows() method:

import polars as pl

df = pl.read_csv("products.csv")

# Convert Polars DataFrame to list of tuples
rows = df.select(["Name", "ProductNumber", "ListPrice"]).rows()

cursor.bulkcopy("dbo.ProductImport", rows, batch_size=5000)
conn.commit()

For complete DataFrame loading patterns, see pandas integration and Polars integration.

Parquet staging

Use Parquet as an intermediate format when migrating data between systems or when your ETL pipeline already produces Parquet files:

import pyarrow.parquet as pq

# Read Parquet file
table = pq.read_table("products.parquet")

# Convert to rows for bulkcopy
rows = [tuple(row) for row in zip(*[col.to_pylist() for col in table.columns])]

cursor.bulkcopy("dbo.ProductImport", rows, batch_size=5000)
conn.commit()

For large Parquet files, read in row groups to keep memory usage constant:

import pyarrow.parquet as pq

parquet_file = pq.ParquetFile("products.parquet")

for batch in parquet_file.iter_batches(batch_size=10000):
    rows = [tuple(row) for row in zip(*[col.to_pylist() for col in batch.columns])]
    cursor.bulkcopy("dbo.ProductImport", rows, batch_size=10000)

conn.commit()

Validate loaded data

After loading, verify row counts and spot-check data:

cursor.execute("SELECT COUNT(*) FROM dbo.ProductImport")
count = cursor.fetchval()
print(f"Total rows: {count}")

cursor.execute("""
    SELECT TOP 5 Name, ProductNumber, ListPrice
    FROM dbo.ProductImport
    ORDER BY Name
""")
for row in cursor:
    print(f"  {row.Name} ({row.ProductNumber}): ${row.ListPrice:.2f}")

For production loads, don't rely on the calling connection's transaction to protect a bulkcopy() call. bulkcopy() opens its own internal connection and commits the copied rows independently, so a conn.rollback() on your main connection can't undo them. Two approaches give you atomicity:

• Set use_internal_transaction=True to wrap each batch in its own transaction. A batch that fails partway rolls back that batch instead of leaving it half-loaded.
• To validate data before promoting it, bulk copy into a staging table, validate it, then move the rows into the target table by using an INSERT ... SELECT inside a transaction on your main connection. Because that INSERT runs on your connection, conn.rollback() undoes it if validation fails.

# Stage the data. bulkcopy() runs on its own connection, so these rows
# persist regardless of the transaction below.
cursor.bulkcopy("dbo.ProductImport_Stage", rows, batch_size=5000)

try:
    cursor.execute("SELECT COUNT(*) FROM dbo.ProductImport_Stage")
    count = cursor.fetchval()

    if count < expected_count:
        raise ValueError(f"Expected {expected_count} rows, got {count}")

    # This INSERT runs on your connection, so it's covered by the transaction.
    cursor.execute("""
        INSERT INTO dbo.ProductImport (Name, ProductNumber, ListPrice)
        SELECT Name, ProductNumber, ListPrice FROM dbo.ProductImport_Stage
    """)
    conn.commit()
except Exception:
    conn.rollback()
    raise

Related content

• Bulk copy operations
• pandas integration
• Polars integration
• Performance tuning
• Transaction management
• Data access and analytics patterns