SELECT

SELECT is used to select data from a table.

ALL indicates that all results are returned.

DISTINCT indicates that the duplicated results are removed.

• The table to be queried must exist. Otherwise, an error is reported.
• WHERE is used to specify the search condition, which can be the arithmetic operator, relational operator, or logical operator.
• GROUP BY is used to specify the grouping field, which can be one or more multiple fields.

Select the order which contains more than 3 pieces of data.

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SELECT  * FROM Orders WHERE units > 3; 

Queries a group of constant data.

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select 'Lily', 'male', 'student', 17;

This clause is used to filter the query results using the WHERE clause.

• The to-be-queried table must exist.
• WHERE filters the records that do not meet the requirements.

Search orders which contain more than 3 pieces and fewer than 10 pieces of data.

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SELECT  * FROM Orders
  WHERE units > 3 and units < 10; 

This clause is used to search for the query results that meet the search condition.

Generally, HAVING and GROUP BY are used together. You can use GROUP BY for grouping and then use HAVING for filtering. Arithmetic operations and aggregate functions are supported in the HAVING clause.

If the filtering condition is subject to the results of GROUP BY, the HAVING clause, rather than the WHERE clause, must be used for search.

Group the student table according to the name field and search for the records in which the maximum score is higher than 95 in the group.

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SELECT name, max(score) FROM student
  GROUP BY name
  HAVING max(score) >95;

This clause is used to group a table based on columns.

Column-based GROUP BY can be categorized into single-column GROUP BY and multi-column GROUP BY.

• Single-column GROUP BY indicates that the GROUP BY clause contains only one column.
• Multi-column GROUP BY indicates that the GROUP BY clause contains multiple columns. The table will be grouped according to all fields in the GROUP BY clause. The records whose fields are the same are grouped into one group.

GroupBy generates update results in the stream processing table.

Group the student table according to the score and name fields and return the grouping results.

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SELECT name,score, max(score) FROM student 
  GROUP BY name,score;

This clause is used to group streams according to expressions.

groupItem can have one or more fields. The fields can be called by string functions, but cannot be called by aggregate functions.

None

Use the substring function to obtain the character string from the name field, group the student table according to the obtained character string, and return each sub character string and the number of records.

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SELECT substring(name,6),count(name) FROM student
  GROUP BY substring(name,6);

• The GROUP BY GROUPING SETS generates a result set equivalent to that generated by multiple simple GROUP BY UNION ALL statements. Using GROUPING SETS is more efficient.
• The ROLLUP and CUBE generate multiple groups based on certain rules and then collect statistics by group.
• The result set generated by CUBE contains all the combinations of values in the selected columns.
• The result set generated by ROLLUP contains the combinations of a certain layer structure in the selected columns.

Values of groupingItem can be Grouping sets(columnName [, columnName]*), Rollup(columnName [, columnName]*), and Cube(columnName [, columnName]*).

None

Return the results generated based on user and product.

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SELECT SUM(amount)
FROM Orders
GROUP BY GROUPING SETS ((user), (product));

This clause filters a table after grouping it using the HAVING clause.

Generally, HAVING and GROUP BY are used together. You can use GROUP BY for grouping and the HAVING for filtering.

• If the filtering condition is subject to the results of GROUP BY, the HAVING clause, rather than the WHERE clause, must be used for search. HAVING and GROUP BY are used together. Use GROUP BY for grouping and the HAVING for filtering.
• Fields used in HAVING, except for those used for aggregate functions, must exist in GROUP BY.
• The arithmetic operation and aggregate function are supported by the HAVING clause.

Group the transactions by num, use the HAVING clause to search for the records in which the maximum value derived from multiplying price with amount is higher than 5000, and return the filtered results.

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SELECT num, max(price*amount) FROM transactions
  WHERE time > '2016-06-01'
  GROUP BY num
  HAVING max(price*amount)>5000;