使用 exists 檢查兩個表中的對應值
要了解兩個表中是否存在對應的值，可以使用 exists 關鍵字。下面的示例演示了如何使用 exists 檢查主表中的值是否出現在子表中：
主表：pnassertbasic_ruku
select pn_code,serialnumber from pnassertbasic_ruku pr left join pnassertruku pu on pr.parent_pnassertruku_id = pu.child_pnassertrukudetails_id where pn_code is not null and serialnumber is not null and date(pr.create_time) = curdate() and pr.warehouseentrynumber <> 'wed3791f0b'
子表：pnassertruku
select pn_code,serialnumber from pnassertbasic_ruku pr left join pnassertruku pu on pr.parent_pnassertruku_id = pu.child_pnassertrukudetails_id where pu.child_pnassertrukudetails_id is null and pr.warehouseentrynumber = 'wed3791f0b'
查詢語句：
select * from ( select pn_Code,serialNumber from pnassertbasic_ruku pr left join pnassertruku pu on pr.parent_pnAssertRuku_id = pu.child_pnAssertRukuDetails_id where pn_Code is not null and serialNumber is not null and DATE(pr.create_time) = CURDATE() and pr.warehouseEntryNumber <> 'WEd3791f0b' ) as temp1 where EXISTS ( select * from ( select pn_Code,serialNumber from pnassertbasic_ruku pr left join pnassertruku pu on pr.parent_pnAssertRuku_id = pu.child_pnAssertRukuDetails_id where pu.child_pnAssertRukuDetails_id is null and pr.warehouseEntryNumber = 'WEd3791f0b' ) as temp2 where temp1.pn_Code = temp2.pn_Code and temp1.serialNumber = temp2.serialNumber )
通過使用 exists 關鍵字，我們可以檢查子表中是否存在與主表中對應的值。如果存在，則查詢結果將返回子表中的匹配記錄；否則，將返回一個空集。
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