Slide rail assembly and slide rail thereof

A slide rail assembly, comprising:

• a first rail comprising a first wall, a second wall and a first middle wall connected between the first wall and the second wall, a passage being defined by the first wall, the second wall and the first middle wall;
• a slide assisting device arranged in the passage of the first rail, and the slide assisting device comprising a first rolling member;
• and
• a second rail movable relative to the first rail, the second rail comprising a third wall, a fourth wall and a second middle wall connected between the third wall and the fourth wall;
• wherein the first middle wall of the first rail is arranged with a first predetermined part bent relative to the first middle wall, the first predetermined part is formed with a first ball groove, the second rail further comprises a first extension wall, the first extension wall comprises a first extension section, a second extension section and a first middle section connected between the first extension section and the second extension section, the first extension section is extended from the third wall of the second rail, and the first middle section is formed with a second ball groove;
• wherein the first rolling member is located between the first ball groove and the second ball groove.