Database Tester#

Validate database operations, data integrity, and migrations.

Test Categories#

CategoryFocusWhenCRUDInsert/Select/Update/Delete correctnessEvery releaseConstraintsPK, FK, UNIQUE, NOT NULL, CHECKSchema changesTransactionsACID compliance, isolation levelsConcurrent featuresMigrationSchema + data migration correctnessVersion upgradesPerformanceSlow queries, index effectivenessPerformance issuesSecuritySQL injection, permissions, encryptionSecurity reviews

Quick Database Validation#

Connect & Inspect#

bash

# MySQL
mysql -h $DB_HOST -u $DB_USER -p$DB_PASS $DB_NAME -e "SHOW TABLES;"

# PostgreSQL
PGPASSWORD=$DB_PASS psql -h $DB_HOST -U $DB_USER -d $DB_NAME -c "\dt"

# SQLite
sqlite3 $DB_FILE ".tables"

Schema Comparison (Migration Verification)#

sql

-- MySQL: Get table structure
SHOW CREATE TABLE users;
DESCRIBE users;

-- PostgreSQL: Get table structure
\d+ users
SELECT column_name, data_type, is_nullable, column_default
FROM information_schema.columns
WHERE table_name = 'users'
ORDER BY ordinal_position;

-- Compare expected vs actual columns
-- After migration, verify:
-- 1. New columns exist with correct type/default
-- 2. Dropped columns are gone
-- 3. Modified columns have new type/constraints
-- 4. Indexes are created/dropped as expected

Constraint Testing#

For each table, verify constraints are enforced:

sql

-- NOT NULL: Insert null into required field → should fail
INSERT INTO users (name, email) VALUES (NULL, 'test@example.com');
-- Expected: ERROR (NOT NULL violation)

-- UNIQUE: Insert duplicate value → should fail
INSERT INTO users (name, email) VALUES ('Test', 'existing@example.com');
-- Expected: ERROR (UNIQUE violation)

-- FOREIGN KEY: Insert invalid reference → should fail
INSERT INTO orders (user_id, total) VALUES (99999, 100.00);
-- Expected: ERROR (FK violation)

-- CHECK constraint
INSERT INTO products (name, price) VALUES ('Test', -10);
-- Expected: ERROR (CHECK violation, price must be >= 0)

-- CASCADE: Delete parent → verify child behavior
DELETE FROM users WHERE id = 1;
-- Verify: orders for user_id=1 are CASCADE deleted/SET NULL per FK rule

Data Migration Testing#

Pre-Migration Checklist#

sql

-- 1. Record baseline counts
SELECT 'users' AS tbl, COUNT(*) AS cnt FROM users
UNION ALL SELECT 'orders', COUNT(*) FROM orders
UNION ALL SELECT 'products', COUNT(*) FROM products;

-- 2. Record sample checksums
SELECT MD5(GROUP_CONCAT(id, name, email ORDER BY id)) AS checksum
FROM users WHERE id BETWEEN 1 AND 100;

-- 3. Record key aggregates
SELECT SUM(total) AS total_revenue FROM orders;
SELECT COUNT(DISTINCT user_id) AS active_users FROM orders;

Post-Migration Verification#

sql

-- 1. Row counts match (or differ by expected amount)
-- 2. Checksums match for unchanged data
-- 3. Aggregates match
-- 4. New columns have correct defaults
-- 5. Transformed data is correct

-- Verify data transformation
SELECT id, old_column, new_column,
  CASE WHEN new_column = EXPECTED_TRANSFORM(old_column)
    THEN 'OK' ELSE 'MISMATCH' END AS status
FROM migrated_table
WHERE status = 'MISMATCH';

Migration Rollback Test#

1. Take snapshot/backup before migration
2. Run migration forward
3. Verify data integrity
4. Run migration rollback
5. Verify data matches pre-migration snapshot

Transaction & ACID Testing#

Atomicity#

sql

-- Start transaction, perform multiple operations, simulate failure
BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
-- Simulate error before commit
ROLLBACK;
-- Verify: both balances unchanged

Isolation Levels#

LevelDirty ReadNon-Repeatable ReadPhantom ReadREAD UNCOMMITTED✅ possible✅ possible✅ possibleREAD COMMITTED❌ prevented✅ possible✅ possibleREPEATABLE READ❌ prevented❌ prevented✅ possibleSERIALIZABLE❌ prevented❌ prevented❌ prevented

Reference: SQL:2016 standard, ISO/IEC 9075

Test procedure: Open two concurrent sessions, verify isolation behavior.

Performance: Slow Query Analysis#

sql

-- MySQL: Enable slow query log
SET GLOBAL slow_query_log = 'ON';
SET GLOBAL long_query_time = 1;  -- seconds

-- MySQL: Find slow queries
SELECT query, exec_count, avg_latency, rows_examined_avg
FROM sys.statements_with_runtimes_in_95th_percentile
ORDER BY avg_latency DESC LIMIT 10;

-- PostgreSQL: Find slow queries
SELECT query, calls, mean_exec_time, total_exec_time
FROM pg_stat_statements
ORDER BY mean_exec_time DESC LIMIT 10;

-- Check missing indexes
EXPLAIN ANALYZE SELECT * FROM orders WHERE user_id = 123;
-- Look for: Seq Scan (bad) vs Index Scan (good)
-- Look for: high rows examined vs rows returned ratio

Index Effectiveness#

sql

-- MySQL: Check index usage
SELECT table_name, index_name, seq_in_index, column_name
FROM information_schema.statistics
WHERE table_schema = DATABASE()
ORDER BY table_name, index_name, seq_in_index;

-- Unused indexes (MySQL 8.0+)
SELECT * FROM sys.schema_unused_indexes;

-- PostgreSQL: Unused indexes
SELECT indexrelname, idx_scan, pg_size_pretty(pg_relation_size(indexrelid))
FROM pg_stat_user_indexes
WHERE idx_scan = 0
ORDER BY pg_relation_size(indexrelid) DESC;

Python Test Script Pattern#

python

"""Database test suite using pytest + direct DB connection.
Reference: PEP 249 (DB-API 2.0)
"""
import pytest
import os

# Use appropriate driver: mysql-connector-python, psycopg2, sqlite3
import mysql.connector  # or psycopg2 for PostgreSQL

@pytest.fixture
def db():
    conn = mysql.connector.connect(
        host=os.getenv("DB_HOST", "localhost"),
        user=os.getenv("DB_USER", "test"),
        password=os.getenv("DB_PASS", "test"),
        database=os.getenv("DB_NAME", "testdb"),
    )
    yield conn
    conn.rollback()  # always rollback test changes
    conn.close()

class TestUserTable:
    def test_insert_valid_user(self, db):
        cur = db.cursor()
        cur.execute(
            "INSERT INTO users (name, email) VALUES (%s, %s)",
            ("Test User", "test@example.com"))
        assert cur.rowcount == 1

    def test_insert_duplicate_email_fails(self, db):
        cur = db.cursor()
        cur.execute(
            "INSERT INTO users (name, email) VALUES (%s, %s)",
            ("User1", "dup@example.com"))
        with pytest.raises(Exception):  # IntegrityError
            cur.execute(
                "INSERT INTO users (name, email) VALUES (%s, %s)",
                ("User2", "dup@example.com"))

    def test_not_null_constraint(self, db):
        cur = db.cursor()
        with pytest.raises(Exception):
            cur.execute(
                "INSERT INTO users (name, email) VALUES (%s, %s)",
                (None, "test@example.com"))

    def test_cascade_delete(self, db):
        cur = db.cursor()
        cur.execute("DELETE FROM users WHERE id = %s", (1,))
        cur.execute("SELECT COUNT(*) FROM orders WHERE user_id = %s", (1,))
        assert cur.fetchone()[0] == 0  # orders cascade deleted

Data Consistency Verification#

After API operations, verify database state:

bash

# Pattern: API call → DB check
# 1. Call API to create order
curl -X POST "$URL/api/orders" -d '{"item_id":1,"qty":2}'

# 2. Verify in database
mysql -e "SELECT * FROM orders ORDER BY id DESC LIMIT 1;" $DB_NAME
mysql -e "SELECT stock FROM products WHERE id = 1;" $DB_NAME
# Verify: stock decreased by 2

References#

For database-specific testing details:

• MySQL specific tests: See references/mysql-tests.md
• PostgreSQL specific tests: See references/postgresql-tests.md