Detailed explanation of the use of custom parameters in MySQL

MySQL variables include system variables and system variables. The learning task this time is user-defined variables. User variables mainly include local variables and session variables.

The declaration method of user-defined variables is as follows: @var_name, where the variable name consists of letters, numbers, ".", "_", and "$". Of course, you can include other characters when referring to a string or identifier (for example: @'my-var', @"my-var", or @my-var).

User-defined variables are session-level variables. The scope of its variables is limited to the client link in which it is declared. When the client disconnects, all its session variables will be released.

User-defined variables are not case-sensitive.

Use the SET statement to declare user-defined variables:

SET @my_var = 1; 
SET @my_var := 1;

When not using set, use := assignment because using = may be considered as a comparison operator.

The following is an example:

Write a SQL query to implement score ranking. If two scores are the same, then the two score ranks should be the same. Note that the next rank number after a tie should be the next consecutive integer value. In other words, there should be no "holes" between the rows and columns.

+—-+——-+
| Id | Score |
+—-+——-+
| 1 | 3.50 |
| 2 | 3.65 |
| 3 | 4.00 |
| 4 | 3.85 |
| 5 | 4.00 |
| 6 | 3.65 |
+—-+——-+

For example, given the Scores table above, your query should produce the following report (sorted by highest score):

+——-+——+
| Score | Rank |
+——-+——+
| 4.00 | 1 |
| 4.00 | 1 |
| 3.85 | 2 |
| 3.65 | 3 |
| 3.65 | 3 |
| 3.50 | 4 |
+——-+——+

Query statement:

select Score, @rank := @rank + (@pre <> (@pre:=Score)) Rank 
from Scores ,(SELECT @rank := 0,@pre := -1) INIT 
ORDER BY Score DESC

Note:

@rank indicates the ranking of grades

@pre indicates the score of the previous person

When the score is different from the previous one, @rank = @rank + 1, otherwise, @rank = rank.

Initialize @rank to 1 and @pre to -1.

The experimental results are:

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