sql - What is the difference between COUNT(*) and COUNT(table.ColumnName)? -


इस सवाल का पहले से ही एक उत्तर है: < / P>

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    COUNT (*) और COUNT (table.ColumnName) के बीच क्या अंतर है?

    मैंने हमेशा सोचा कि यह COUNT के लिए एक पैरामीटर के रूप में एक स्तंभ का नाम डालने के लिए तेज़ है, लेकिन अब जब मैं इसके बारे में सोचता हूं तो मुझे वाकई पता नहीं है क्योंकि * इसका अर्थ है कि फ़ंक्शन पंक्तियों की गिनती करता है, इसलिए शायद कोई अंतर नहीं है? या हो सकता है कि एक विशिष्ट स्तंभ नाम देकर भी प्रक्रिया को धीमा कर दे?

    इन दोनों के बीच अंतर नहीं है (मुख्यतः ) प्रदर्शन। वे अलग-अलग चीज़ों की गणना करते हैं:

    COUNT (*) आपकी तालिका में पंक्तियों की गणना करता है।

    COUNT (कॉलम) एक कॉलम में प्रविष्टियां - null मानों की अनदेखी कर रही हैं।

    बेशक इन दोनों के बीच प्रदर्शन अंतर होगा, लेकिन ये उम्मीद की जानी चाहिए कि वे अलग-अलग चीज़ें कर रहे हैं विशेषकर जब कॉलम null -values ​​की अनुमति देता है, तो क्वेरी उस कॉलम पर से अधिक समय लगेगी जो (या COUNT (*) )।

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