String comparison size in MySQL (date string comparison problem)

String comparison size in MySQL (date string comparison problem)

In the database, when comparing the size of strings with numbers and non-numbers, If the lengths of the two strings are equal, then the two strings will comparison at the same position.Character , if the character is a number, it will be compared directly. If the character is not a number, it will be converted to ascii code for comparison.Compare.

As mentioned above, problems arise when strings are used to compare time-formatted data for time-dimension comparisons.When querying as the following sql

 select * FROM work_day_content as wdc WHERE wdc.work_day >= '2019-09-30 00:00:00' AND wdc.work_day<= '2019-10-06 00:00:00'

Figure 1-1

As shown in Figure 1-1, no conditions were found

When using the date function to convert a string to a date

select * FROM work_day_content as wdc where date(wdc.work_day) >= date('2019-09-30 00:00:00' ) AND date(wdc.work_day)<= date('2019-09-30 00:00:00' )

Figure 1-2

As shown in Figure 1-2, the qualified data was found

Analyze why the first sql statement did not query the content.Since the field is stored in the form of a string, the comparison of the size is performed in the form of a string at this time.

The size of the string comparison is from left to right.When matching strings of different lengths.There is no size difference after the shorter strings are compared.longer strings are larger.As shown in the following sql

select '2019-09-30' < '2019-09-30 00:00:00'

It is found that the **string '2019-09-30 00:00:00'** is larger at this time.Therefore, the previous sql did not find the data dated 2019-09-30

Therefore.When using strings to represent time, you need to use the time function for comparison.

The time functions provided by sql are: date(), str_to_date()

In addition, in mysql, when adding, subtracting, multiplying and dividing between string types, the part of the number that starts with a number will be intercepted for operation. If there is no number in front of the string, the only value that can be intercepted is0, then the result of addition and subtraction is 0, and the result of multiplication and division is NULL.

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