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Shewani has recently started working in MySQL. Help her in understanding the difference between the following :(i) Where and having clause(ii) Count(column_name) and count(*) |
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Answer» (i) Where clause is used to show data set for a table based on a condition and having clause is used to put condition on the result set that comes after using Group by clause. (ii) COUNT(*) returns the number of items in a group, including NULL values and duplicates. COUNT(expression) evaluates expression for each row in a group and returns the number of non null values. Candidate Key – A Candidate Key can be any column or a combination of columns that can qualify as unique key in database. There can be multiple Candidate Keys in one table. Each Candidate Key can qualify as Primary Key. Primary Key – A Primary Key is a column or a combination of columns that uniquely identify a record. Only one Candidate Key can be Primary Key. A table can have multiple Candidate Keys that are unique as single column or combined multiple columns to the table. They are all candidates for Primary Key. |
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