diff --git a/src/main/services/database/postgresql.ts b/src/main/services/database/postgresql.ts
index 569cf01..b0eba23 100644
--- a/src/main/services/database/postgresql.ts
+++ b/src/main/services/database/postgresql.ts
@@ -11,6 +11,168 @@ const log = createLogger('PostgreSqlService')
 
 export type { PostgreSqlConfig } from '../../types/database.types'
 
+/**
+ * SQL keywords that should NOT be double-quoted during identifier preprocessing.
+ * PostgreSQL lowercases unquoted identifiers, but SSMA-migrated databases
+ * have uppercase column names that require double-quoting to preserve case.
+ */
+const SQL_KEYWORDS = new Set([
+  // DML
+  'SELECT', 'FROM', 'WHERE', 'AND', 'OR', 'NOT', 'IN', 'IS', 'NULL',
+  'INSERT', 'INTO', 'VALUES', 'UPDATE', 'SET', 'DELETE',
+  // Ordering & limiting
+  'ORDER', 'BY', 'ASC', 'DESC', 'LIMIT', 'OFFSET',
+  'FETCH', 'NEXT', 'ROWS', 'ONLY',
+  // Joins
+  'JOIN', 'LEFT', 'RIGHT', 'INNER', 'OUTER', 'CROSS', 'FULL', 'ON',
+  // Set operations
+  'UNION', 'ALL', 'INTERSECT', 'EXCEPT',
+  // Grouping
+  'GROUP', 'HAVING', 'DISTINCT',
+  // DDL
+  'CREATE', 'ALTER', 'DROP', 'TABLE', 'INDEX', 'COLUMN',
+  'ADD', 'MODIFY', 'RENAME', 'TO',
+  // PostgreSQL specific
+  'CONFLICT', 'DO', 'NOTHING', 'EXCLUDED', 'RETURNING',
+  'MERGE', 'USING', 'MATCHED', 'WHEN', 'THEN', 'ELSE', 'END',
+  'TARGET', 'SOURCE',
+  // Functions
+  'COUNT', 'SUM', 'AVG', 'MIN', 'MAX', 'EXISTS',
+  'CURRENT_TIMESTAMP', 'NOW', 'GETDATE',
+  'COALESCE', 'NULLIF', 'CAST', 'AS',
+  // Transaction
+  'BEGIN', 'COMMIT', 'ROLLBACK', 'SAVEPOINT',
+  // Types & values
+  'TRUE', 'FALSE', 'DEFAULT', 'PRIMARY', 'KEY',
+  'REFERENCES', 'FOREIGN', 'CONSTRAINT', 'UNIQUE', 'CHECK',
+  'CASE', 'BETWEEN', 'LIKE', 'ILIKE', 'ANY', 'SOME',
+  // Common
+  'IF', 'WITH', 'RECURSIVE', 'OVER', 'PARTITION', 'WINDOW',
+  'ROW', 'FIRST', 'AFTER', 'BEFORE'
+])
+
+/**
+ * Prepare SQL for PostgreSQL execution by quoting unquoted identifiers.
+ *
+ * PostgreSQL lowercases unquoted identifiers, but SSMA-migrated tables
+ * have uppercase column names (e.g., "UserName", "ID") that require
+ * double-quoting to preserve case.
+ *
+ * This function:
+ * - Preserves string literals ('...')
+ * - Preserves already-quoted identifiers ("...")
+ * - Preserves parameter placeholders ($1, $2, ...)
+ * - Preserves SQL keywords
+ * - Double-quotes remaining identifiers
+ */
+export function prepareSql(sql: string): string {
+  const result: string[] = []
+  let i = 0
+  const len = sql.length
+
+  while (i < len) {
+    const ch = sql[i]
+
+    // Skip whitespace
+    if (/\s/.test(ch)) {
+      result.push(ch)
+      i++
+      continue
+    }
+
+    // Skip single-line comments (--)
+    if (ch === '-' && i + 1 < len && sql[i + 1] === '-') {
+      while (i < len && sql[i] !== '\n') {
+        result.push(sql[i++])
+      }
+      continue
+    }
+
+    // Preserve string literals ('...')
+    if (ch === "'") {
+      result.push(ch)
+      i++
+      while (i < len) {
+        if (sql[i] === "'") {
+          result.push(sql[i++])
+          // Handle escaped quotes ('')
+          if (i < len && sql[i] === "'") {
+            result.push(sql[i++])
+          } else {
+            break
+          }
+        } else {
+          result.push(sql[i++])
+        }
+      }
+      continue
+    }
+
+    // Preserve already-quoted identifiers ("...")
+    if (ch === '"') {
+      result.push(ch)
+      i++
+      while (i < len && sql[i] !== '"') {
+        result.push(sql[i++])
+      }
+      if (i < len) {
+        result.push(sql[i++])
+      }
+      continue
+    }
+
+    // Preserve parameter placeholders ($N)
+    if (ch === '$') {
+      result.push(ch)
+      i++
+      while (i < len && /\d/.test(sql[i])) {
+        result.push(sql[i++])
+      }
+      continue
+    }
+
+    // Preserve @param placeholders
+    if (ch === '@') {
+      result.push(ch)
+      i++
+      while (i < len && /\w/.test(sql[i])) {
+        result.push(sql[i++])
+      }
+      continue
+    }
+
+    // Preserve ? placeholders
+    if (ch === '?') {
+      result.push(ch)
+      i++
+      continue
+    }
+
+    // Collect word tokens (identifiers and keywords)
+    if (/[a-zA-Z_]/.test(ch)) {
+      let word = ''
+      while (i < len && /\w/.test(sql[i])) {
+        word += sql[i++]
+      }
+
+      // Check if it's a SQL keyword (case-insensitive)
+      if (SQL_KEYWORDS.has(word.toUpperCase())) {
+        result.push(word)
+      } else {
+        // Quote the identifier to preserve case
+        result.push(`"${word}"`)
+      }
+      continue
+    }
+
+    // Everything else (operators, punctuation, numbers): pass through
+    result.push(ch)
+    i++
+  }
+
+  return result.join('')
+}
+
 export class PostgreSqlService implements IDatabaseService {
   /** Database type identifier */
   readonly type: DatabaseType = 'postgresql'
@@ -95,13 +257,15 @@ export class PostgreSqlService implements IDatabaseService {
       throw new Error('Not connected to PostgreSQL. Call connect() first.')
     }
 
-    const sqlPreview = sql.substring(0, 100)
+    // Quote unquoted identifiers to preserve case for SSMA-migrated columns
+    const preparedSql = prepareSql(sql)
+    const sqlPreview = preparedSql.substring(0, 100)
     const paramCount = params?.length ?? 0
 
     try {
       const { result: queryResult } = await trackDuration(
         async () => {
-          const result = await this.pool!.query(sql, params)
+          const result = await this.pool!.query(preparedSql, params)
 
           // Extract column names from fields
           const columns = result.fields ? result.fields.map((field) => field.name) : []
@@ -141,10 +305,11 @@ export class PostgreSqlService implements IDatabaseService {
 
       for (let i = 0; i < queries.length; i++) {
         const { sql, params } = queries[i]
-        await client.query(sql, params)
+        const preparedSql = prepareSql(sql)
+        await client.query(preparedSql, params)
         log.debug('Transaction query executed', {
           index: i,
-          sqlPreview: sql.substring(0, 100)
+          sqlPreview: preparedSql.substring(0, 100)
         })
       }
 
diff --git a/tests/unit/postgresql.test.ts b/tests/unit/postgresql.test.ts
index 7dcaa00..8b6d9c5 100644
--- a/tests/unit/postgresql.test.ts
+++ b/tests/unit/postgresql.test.ts
@@ -4,7 +4,7 @@
  */
 
 import { describe, it, expect, beforeEach } from 'vitest'
-import { PostgreSqlService } from '@main/services/database/postgresql'
+import { PostgreSqlService, prepareSql } from '@main/services/database/postgresql'
 
 const mockConfig = {
   host: 'localhost',
@@ -68,3 +68,146 @@ describe('PostgreSqlService Unit Tests', () => {
     })
   })
 })
+
+describe('prepareSql', () => {
+  it('should quote unquoted column names in SELECT', () => {
+    const sql = 'SELECT ID, UserName, UserType FROM "dbo"."BIPUsers"'
+    const result = prepareSql(sql)
+    expect(result).toBe('SELECT "ID", "UserName", "UserType" FROM "dbo"."BIPUsers"')
+  })
+
+  it('should quote column names in WHERE clause', () => {
+    const sql = 'WHERE UserName = $1 AND Password = $2'
+    const result = prepareSql(sql)
+    expect(result).toBe('WHERE "UserName" = $1 AND "Password" = $2')
+  })
+
+  it('should quote column names in INSERT', () => {
+    const sql = 'INSERT INTO "dbo"."BIPUsers" (UserName, Password, UserType) VALUES ($1, $2, $3)'
+    const result = prepareSql(sql)
+    expect(result).toBe(
+      'INSERT INTO "dbo"."BIPUsers" ("UserName", "Password", "UserType") VALUES ($1, $2, $3)'
+    )
+  })
+
+  it('should quote column names in UPDATE SET', () => {
+    const sql = 'UPDATE "dbo"."BIPUsers" SET UserType = $1 WHERE UserName = $2'
+    const result = prepareSql(sql)
+    expect(result).toBe('UPDATE "dbo"."BIPUsers" SET "UserType" = $1 WHERE "UserName" = $2')
+  })
+
+  it('should quote column names in DELETE', () => {
+    const sql = 'DELETE FROM "dbo"."BIPUsers" WHERE UserName = $1'
+    const result = prepareSql(sql)
+    expect(result).toBe('DELETE FROM "dbo"."BIPUsers" WHERE "UserName" = $1')
+  })
+
+  it('should quote column names in ORDER BY', () => {
+    const sql = 'SELECT UserName FROM "dbo"."BIPUsers" ORDER BY UserName'
+    const result = prepareSql(sql)
+    expect(result).toBe('SELECT "UserName" FROM "dbo"."BIPUsers" ORDER BY "UserName"')
+  })
+
+  it('should not quote SQL keywords', () => {
+    const sql = 'SELECT ID FROM "dbo"."BIPUsers" WHERE UserName = $1'
+    const result = prepareSql(sql)
+    expect(result).not.toContain('"SELECT"')
+    expect(result).not.toContain('"FROM"')
+    expect(result).not.toContain('"WHERE"')
+    expect(result).not.toContain('"AND"')
+  })
+
+  it('should not quote already-quoted identifiers', () => {
+    const sql = 'SELECT "ID" FROM "dbo"."BIPUsers"'
+    const result = prepareSql(sql)
+    expect(result).toBe('SELECT "ID" FROM "dbo"."BIPUsers"')
+  })
+
+  it('should preserve string literals', () => {
+    const sql = "WHERE Status = 'active'"
+    const result = prepareSql(sql)
+    expect(result).toBe('WHERE "Status" = \'active\'')
+  })
+
+  it('should preserve string literals with escaped quotes', () => {
+    const sql = "WHERE UserName = 'O''Brien'"
+    const result = prepareSql(sql)
+    expect(result).toBe('WHERE "UserName" = \'O\'\'Brien\'')
+  })
+
+  it('should preserve $N parameter placeholders', () => {
+    const sql = 'WHERE UserName = $1 AND Password = $2'
+    const result = prepareSql(sql)
+    expect(result).toContain('$1')
+    expect(result).toContain('$2')
+  })
+
+  it('should handle COUNT(*) correctly', () => {
+    const sql = 'SELECT COUNT(*) as count FROM "dbo"."BIPUsers" WHERE UserName = $1'
+    const result = prepareSql(sql)
+    expect(result).toBe(
+      'SELECT COUNT(*) as count FROM "dbo"."BIPUsers" WHERE "UserName" = $1'
+    )
+  })
+
+  it('should quote underscore-containing column names', () => {
+    const sql = 'SELECT ERP_URL, ERP_Username, ERP_Password FROM "dbo"."BIPUsers"'
+    const result = prepareSql(sql)
+    expect(result).toBe(
+      'SELECT "ERP_URL", "ERP_Username", "ERP_Password" FROM "dbo"."BIPUsers"'
+    )
+  })
+
+  it('should handle ON CONFLICT DO UPDATE SET with EXCLUDED', () => {
+    const sql =
+      'INSERT INTO "dbo"."Materials" (MaterialCode, ManagerName) VALUES ($1, $2) ON CONFLICT ("MaterialCode") DO UPDATE SET "ManagerName" = EXCLUDED."ManagerName"'
+    const result = prepareSql(sql)
+    expect(result).toBe(
+      'INSERT INTO "dbo"."Materials" ("MaterialCode", "ManagerName") VALUES ($1, $2) ON CONFLICT ("MaterialCode") DO UPDATE SET "ManagerName" = EXCLUDED."ManagerName"'
+    )
+  })
+
+  it('should handle CURRENT_TIMESTAMP without quoting', () => {
+    const sql = "INSERT INTO t (OperationTime) VALUES (CURRENT_TIMESTAMP)"
+    const result = prepareSql(sql)
+    expect(result).toBe('INSERT INTO "t" ("OperationTime") VALUES (CURRENT_TIMESTAMP)')
+  })
+
+  it('should handle LIMIT OFFSET without quoting', () => {
+    const sql = 'SELECT UserName FROM "dbo"."BIPUsers" LIMIT 10 OFFSET 20'
+    const result = prepareSql(sql)
+    expect(result).toBe('SELECT "UserName" FROM "dbo"."BIPUsers" LIMIT 10 OFFSET 20')
+  })
+
+  it('should return empty string for empty input', () => {
+    expect(prepareSql('')).toBe('')
+  })
+
+  it('should handle full BIPUsersDAO authenticate query', () => {
+    const sql = `
+        SELECT ID, UserName, UserType
+        FROM "dbo"."BIPUsers"
+        WHERE UserName = $1 AND Password = $2
+      `
+    const result = prepareSql(sql)
+    expect(result).toContain('"ID"')
+    expect(result).toContain('"UserName"')
+    expect(result).toContain('"UserType"')
+    expect(result).toContain('"Password"')
+    expect(result).toContain('$1')
+    expect(result).toContain('$2')
+    expect(result).toContain('"dbo"."BIPUsers"')
+  })
+
+  it('should handle full BIPUsersDAO userExists query', () => {
+    const sql = `
+        SELECT COUNT(*) as count
+        FROM "dbo"."BIPUsers"
+        WHERE UserName = $1
+      `
+    const result = prepareSql(sql)
+    expect(result).toContain('COUNT(*)')
+    expect(result).toContain('as count')
+    expect(result).toContain('"UserName"')
+  })
+})