![]() ORDER BY COUNT(*) Code language: SQL (Structured Query Language) ( sql ) The following statement returns the customers and their orders: SELECT See the following orders and customers tables: MySQL CASE expression examples A) Using CASE expression in the SELECT clause example If the CASE expression is used in a numeric context, it returns the result as an integer, a decimal, or a real value. For example, if the CASE expression is used in the character string context, it returns the result as a character string. The CASE expression returns a result whose data type depends on the context where it is used. In case the ELSE clause is not available, then the CASE expression returns NULL. Otherwise, it returns the result specified in the ELSE clause. CASE returns the corresponding result in the THEN clause. ![]() In this syntax, the CASE evaluates expressions specified in the WHEN clauses. The following shows the syntax of a searched CASE expression: CASE The CASE compares the value with values in the WHEN clauses for equality, you cannot use it with NULL because NULL = NULL returns false. In this syntax, CASE matches the value with the value1, value2, etc., for equality and return the corresponding result1, result2,… If the value does not equal to any value1, value2, … CASE returns the result in the ELSE clause if the ELSE clause is specified. The following illustrates the syntax of a simple CASE expression: CASE valueĮND Code language: SQL (Structured Query Language) ( sql ) Note that MySQL has a CASE statement that you can use only in stored programs such as stored procedures, stored functions, events and triggers, which is not the CASE expression covered in this tutorial. The CASE expression has two forms: simple CASE and searched CASE. Generally speaking, you can use the CASE expression anywhere that allows a valid expression e.g., SELECT, WHERE and ORDER BY clauses. ![]() MySQL CASE expression is a control flow structure that allows you to add if-else logic to a query. Coordinate this activity with the database administrators (DBAs) and make a backup of the original statistics.Summary: in this tutorial, you will learn how to use the MySQL CASE expression to add if-else logic to queries. That might, however, also lead to unwanted side effects. My personal recommendation goes even further: after every index change, update the statistics for the base table and all its indexes. Such statistics will enable Oracle Database to correctly decide when to use the index. For this reason, the Oracle documentation recommends updating the table statistics after creating a function-based index:Īfter creating a function-based index, collect statistics on both the index and its base table using the DBMS_STATS package. Although the Oracle database collects the index statistics for new indexes automatically ( since release 10 g), it does not update the table statistics. Statistics for a function-based index (FBI) are also kept on table level as virtual columns. These figures are reused if a column is part of multiple indexes. The Oracle database maintains the information about the number of distinct column values as part of the table statistics. There are no dedicated operations or keywords for function-based indexes. The database traverses the B-tree and follows the leaf node chain. It is a regular INDEX RANGE SCAN as described in Chapter 1. Index Cond: (upper((last_name)::text) = 'WINAND'::text) Recheck Cond: (upper((last_name)::text) = 'WINAND'::text) |*2 | INDEX RANGE SCAN | EMP_UP_NAME | 40 | 1 |Ģ - access(UPPER("LAST_NAME")='WINAND') PostgreSQL QUERY PLAN | 1 | TABLE ACCESS BY INDEX ROWID| EMPLOYEES | 100 | 41 | See “ Full Table Scan” to see why that might make sense. This causes the optimizer to ignore the index and do a TBSCAN. When using UPPER('winand'), the optimizer does a gross misestimation and expects 4% of the table rows to be selected. The query was changed to WHERE UPPER(last_name) = 'WINAND' (no UPPER on the right hand side) to get the expected result. ![]() From the database’s perspective, that’s something entirely different.Ģ | FETCH EMPLOYEES | 1 of 1 (100.00%) | 13ģ | IXSCAN EMP_UP_NAME | 1 of 10000 (. Although there is an index on LAST_NAME, it is unusable-because the search is not on LAST_NAME but on UPPER(LAST_NAME). It is a return of our old friend the full table scan. Predicate Information (identified by operation id):ġ - filter(UPPER("LAST_NAME")='WINAND') PostgreSQL QUERY PLANįilter: (upper((last_name)::text) = 'WINAND'::text) |* 1 | TABLE ACCESS FULL| EMPLOYEES | 10 | 477 |
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