sql 如果字段为空就用另一个字段

您可以使用 COALESCE() 函数 或 CASE WHEN 表达式来处理这个逻辑：

方案一：使用 COALESCE() 函数（推荐）

SELECT
    bpi.batch_id AS batchId,
    bpi.batch_name AS batchName,
    bpi.plate_name AS plateName,
    bpi.category AS productType,
    bpi.product_id AS productId,
    bpi.product_name AS productName,
    bpi.opening_time AS openingTime,
    bpi.section_id AS sectionId,
    bpi.section_name AS sectionName,
    bpi.company_id AS companyId,
    bpi.company_name AS supplier,
    bpi.purchase_type AS purchaseType,
    COALESCE(bpi.winning_bid_amount, bpi.bid_price) AS winningBidAmount
FROM bid_project_info bpi

方案二：使用 CASE WHEN 表达式

SELECT
    bpi.batch_id AS batchId,
    bpi.batch_name AS batchName,
    bpi.plate_name AS plateName,
    bpi.category AS productType,
    bpi.product_id AS productId,
    bpi.product_name AS productName,
    bpi.opening_time AS openingTime,
    bpi.section_id AS sectionId,
    bpi.section_name AS sectionName,
    bpi.company_id AS companyId,
    bpi.company_name AS supplier,
    bpi.purchase_type AS purchaseType,
    CASE 
        WHEN bpi.winning_bid_amount IS NULL THEN bpi.bid_price
        ELSE bpi.winning_bid_amount
    END AS winningBidAmount
FROM bid_project_info bpi

方案三：使用 IFNULL() 函数（MySQL、SQLite等）

SELECT
    bpi.batch_id AS batchId,
    bpi.batch_name AS batchName,
    bpi.plate_name AS plateName,
    bpi.category AS productType,
    bpi.product_id AS productId,
    bpi.product_name AS productName,
    bpi.opening_time AS openingTime,
    bpi.section_id AS sectionId,
    bpi.section_name AS sectionName,
    bpi.company_id AS companyId,
    bpi.company_name AS supplier,
    bpi.purchase_type AS purchaseType,
    IFNULL(bpi.winning_bid_amount, bpi.bid_price) AS winningBidAmount
FROM bid_project_info bpi

方案四：使用 ISNULL() 函数（SQL Server）

SELECT
    bpi.batch_id AS batchId,
    bpi.batch_name AS batchName,
    bpi.plate_name AS plateName,
    bpi.category AS productType,
    bpi.product_id AS productId,
    bpi.product_name AS productName,
    bpi.opening_time AS openingTime,
    bpi.section_id AS sectionId,
    bpi.section_name AS sectionName,
    bpi.company_id AS companyId,
    bpi.company_name AS supplier,
    bpi.purchase_type AS purchaseType,
    ISNULL(bpi.winning_bid_amount, bpi.bid_price) AS winningBidAmount
FROM bid_project_info bpi

📌 各方案对比

方案	函数	适用数据库	特点
方案一	COALESCE()	所有主流数据库（标准SQL）	最通用，可处理多个参数，返回第一个非NULL值
方案二	CASE WHEN	所有数据库	最灵活，可处理复杂条件
方案三	IFNULL()	MySQL、SQLite、MariaDB	简洁，但只适用于特定数据库
方案四	ISNULL()	SQL Server	SQL Server专用

💡 推荐与说明

推荐使用方案一的 COALESCE() 函数，原因如下：

1. 标准化 ：COALESCE() 是 SQL 标准函数，在所有主流数据库（MySQL、PostgreSQL、Oracle、SQL Server、SQLite等）中都可用

2. 可扩展性 ：可以处理多个字段的优先级，例如：

   COALESCE(bpi.winning_bid_amount, bpi.bid_price, bpi.estimate_price, 0) AS winningBidAmount

   （依次尝试 winning_bid_amount、bid_price、estimate_price，都为空则返回 0）

3. 简洁性 ：代码比 CASE WHEN 更简洁

🔍 完整查询示例

SELECT
    bpi.batch_id AS batchId,
    bpi.batch_name AS batchName,
    bpi.plate_name AS plateName,
    bpi.category AS productType,
    bpi.product_id AS productId,
    bpi.product_name AS productName,
    bpi.opening_time AS openingTime,
    bpi.section_id AS sectionId,
    bpi.section_name AS sectionName,
    bpi.company_id AS companyId,
    bpi.company_name AS supplier,
    bpi.purchase_type AS purchaseType,
    -- 如果winning_bid_amount为空，则使用bid_price
    COALESCE(bpi.winning_bid_amount, bpi.bid_price) AS winningBidAmount
FROM bid_project_info bpi
WHERE ...  -- 可以添加其他查询条件
ORDER BY ...  -- 可以添加排序

⚠️ 注意事项

1. 确保 bid_price 字段存在于 bid_project_info 表中

2. 如果 bid_price 也可能为 NULL，可以使用嵌套的 COALESCE() 或设置默认值：

   COALESCE(bpi.winning_bid_amount, bpi.bid_price, 0) AS winningBidAmount

3. 数据类型要兼容：winning_bid_amount 和 bid_price 应该是相同或兼容的数据类型（如都是 DECIMAL、FLOAT 等）